What a customer costs, and how fast you get it back

Decide course — make the call

• Track: Marketing
• Difficulty: Beginner · 3/8
• Read time: ~18 min, then you make the call
• The concept: CAC and CAC-payback period
• Scored decision — one defensible answer, reasoning shown. You pick the call; every wrong option shows what it costs you. This is a Decide course: you make a judgment, you don’t run code, and it is not a certification.
• ▶ Interactive model — a calculator on this page turns your own numbers into the decision, so you can stress-test the call yourself.

1. Before you start

Customer acquisition cost (CAC) is the money you spend to win one new customer: take everything sales and marketing spent over a period and divide it by the number of new customers that period brought in. Spend $10,000 and sign 100 new customers, and your CAC is $100 — that is what each new logo cost you, on average.

CAC-payback period is how long that customer takes to pay you back. If a customer throws off $25 a month in gross margin, a $100 CAC is repaid in $100 ÷ $25 = 4 months. After that, you are ahead; before that, you are still in the hole on them.

Three honest statements before you start:

• This is a Decide course. You read a situation, learn the idea, and make a call. You do not write or run any code.
• The companies below, Meridian Analytics and Larkspur in section 7, are composite — invented firms built from ordinary figures so the arithmetic is clean. No number is a claim about any real company.
• This is not a certification. It proves, to you, that you can defend a spend-and-payback call.

You need only arithmetic. The hard part is judgment: which spend counts, and how fast is fast enough.

2. The Situation

Meridian Analytics sells a $80-a-month analytics dashboard to small businesses. Growth looks great — last quarter it added 300 customers — and the founder wants to triple the ad budget on the strength of it. The finance lead is uneasy: “we spent $180,000 to get those 300, and I don’t know how long it takes to earn that back.”

You have to make the call, and the number that decides it is not the growth rate everyone is celebrating — it is how much each customer cost and how many months of margin it takes to recover that cost. Get it wrong and tripling spend triples a hidden loss.

3. What you’ll be able to do

After this course you will be able to:

• Compute a customer acquisition cost from a spend figure and a new-customer count, and say which spend belongs in it.
• Compute a CAC-payback period in months from CAC and the gross margin a customer throws off, and judge it against a threshold.
• Tell paid CAC from blended CAC, and spot when a healthy-looking blended number is hiding an underwater paid channel.
• Catch the case where payback looks fine but the customer churns out before it ever pays off — and change the call.

4. Prerequisites & time box

Prerequisites: arithmetic, and the idea of gross margin (price minus the cost of serving that unit — covered in section 6). No spreadsheet, no marketing background. If you have not met a profit statement, the Decide course read-a-pnl is a gentle warm-up but is not required.

Time box: about 18 minutes of reading (measured), plus your own thinking time on the call. That is under the 25-minute cap for a concept course.

Difficulty: 3 / 8 — a new-manager decision: a couple of interacting factors and one real judgment call, where the obvious answer is often the trap.

Free-tier honesty: no signup, no GPU, nothing to install.

5. The case & where the numbers come from

Meridian Analytics is a composite software-as-a-service company: a subscription analytics tool, with a cost structure built from ordinary figures a small SaaS business would recognise, chosen for clean arithmetic and not drawn from or claimed about any real firm. The definitions — customer acquisition cost, gross margin, payback period, churn — are standard and cited in section 11. Every figure below is an in-course assumption; every later number is computed from these.

Item	Figure
Subscription price per customer	$80 / month
Gross margin on the subscription	80%
New customers added last quarter	300
Sales & marketing spend last quarter	$180,000
Of those 300, credited to paid campaigns	180
Of those 300, arrived organically (referral, inbound)	120
Average customer lifetime (before they cancel)	30 months
The team’s payback threshold (“healthy if under…“)	12 months

The 80% gross margin means each $80 subscription leaves $64 a month after the cost of serving it (hosting, support, payment fees). That $64 — not the full $80 — is what pays back acquisition cost, and it drives everything below.

6. The Concepts

Customer acquisition cost

Customer acquisition cost is the sales-and-marketing spend of a period divided by the new customers that period produced. For Meridian last quarter: $180,000 ÷ 300 = $600. Each new customer cost, on average, $600 to win.

That is the blended figure — every new customer, over the whole spend. Hold on to it; the next concept refines it. First, what does $600 buy? A customer paying $80 a month at 80% margin returns $64 a month in gross margin. So before we even ask “how long,” note that $600 is about nine months of that customer’s margin — real money, not a rounding error.

(An interactive calculator sits here — enter your own spend, customers, price, margin, and threshold, and watch the CAC, the payback, and the decision change.)

At the defaults this returns CAC $600, monthly margin $64, and a payback of 9.4 months — inside the 12-month line. Drop the threshold to 8 months, or push CAC up, and the same model flips to a reject.

CAC-payback period

CAC-payback period is CAC divided by the gross margin one customer throws off per month: how many months of that customer’s margin it takes to recover what you spent to win them. For Meridian: $600 ÷ $64 = 9.4 months. Set against the team’s 12-month threshold, that is healthy — a customer is fully paid back well inside a year, and every month after is profit.

Why gross margin and not revenue? Because the $80 does not all come back to you — $16 of it goes to serving the customer. Paying CAC back out of the full $80 would overstate how fast you recover; the honest number is the $64 that actually lands. Use revenue and a 9.4-month payback would read as 7.5 months, and you would fund a channel that is slower than you think.

Paid CAC versus blended CAC

Here is the number the founder was celebrating and the one the finance lead should have asked for. Blended CAC spreads all spend over all new customers, including the ones who cost nothing. Paid CAC counts only the customers the spend actually bought.

Of Meridian’s 300 new customers, 120 arrived organically — referrals and inbound, not paid for. Only 180 came from the paid campaigns that ate the $180,000. So:

• Blended CAC: $180,000 ÷ 300 = $600 → payback $600 ÷ $64 = 9.4 months.
• Paid CAC: $180,000 ÷ 180 = $1,000 → payback $1,000 ÷ $64 = 15.6 months.

The blended number ($600, 9.4 months) looks healthy. The paid channel alone costs $1,000 a customer and takes 15.6 months to pay back — past the 12-month line. If the founder triples the ad budget, the organic 120 do not triple with it; the spend buys more paid customers at the paid CAC. Judge new spend on paid CAC, not blended. Blended CAC is the right number for “how are we doing overall”; paid CAC is the right number for “should we spend more.”

Payback versus customer lifetime

A fast payback is only good news if the customer is still around to deliver it. Meridian’s customers last, on average, 30 months before they cancel. Over that life each returns 30 × $64 = $1,920 of gross margin, against a $600 blended CAC — the customer pays back in 9.4 months and then hands you 20 more months of margin. Comfortable.

Payback is a speed limit, not the whole story: it tells you how fast you recover, but you must check it against how long the customer stays. A 9-month payback on a customer who leaves at month 10 barely breaks even; the same payback on a customer who stays three years is a good business. Always read payback next to lifetime — the call can flip on the second number even when the first looks fine, as the next section shows.

7. Your Call

You have seen how CAC, payback, the paid-versus-blended distinction, and lifetime decide Meridian’s call. Now a different one lands on your desk.

Larkspur is a consumer language-learning app — a different company in a different corner of software from Meridian’s B2B analytics tool. It charges $15 a month at an 85% gross margin, so each subscriber returns $12.75 a month. Last month a paid-ads channel spent $60,000 and brought in 500 new paying subscribers; a further 250 signed up organically. The growth lead wants to scale the paid channel hard. The catch: consumer apps churn fast, and Larkspur’s subscribers last, on average, only 10 months before they cancel. Your job is a go/no-go on scaling the paid channel, using the same tools.

How this differs from the taught case (the transfer): this is a different company and sector (a B2C consumer app, not Meridian’s B2B analytics firm), the figures are different so the arithmetic must be redone, it is a different decision (a go/no-go on scaling a channel, not judging one quarter as healthy), and it adds a binding constraint — a short 10-month customer lifetime that the taught case never had to worry about. The core concept is the same: CAC and CAC-payback, read against lifetime, decide the call.

8. Self-check

Before you write the memo, make sure you can say each of these in one line:

• Why is $80 (blended) the wrong CAC to judge more ad spend, and $120 (paid) the right one?
• Why does payback use the $12.75 of gross margin and not the $15 price?
• What single second number turns a 9.4-month payback that “passes” into a channel you would not scale?

If any is fuzzy, reread section 6 — CAC, payback, paid-versus-blended, and payback-versus- lifetime are the whole course.

9. Stretch

Push the decision further on your own:

• Back at Meridian: the founder triples ad spend to $540,000 and it buys 540 paid customers (organic stays flat at 120). What is the new paid CAC and its payback, and does the channel still clear 12 months?
• For Larkspur: what would the monthly gross margin have to become for the paid channel to return twice its CAC over a 10-month life? (The genuinely harder one: solve 10 × margin = 2 × $120, then back out the price at 85% margin.)
• Write the one sentence you would say to a founder who wants to triple spend because “growth is up and to the right.”

10. Ship it — your decision memo

Write a one-page memo to Larkspur’s founder. State the call (do not scale the paid channel hard on these numbers: paid CAC is $120, payback 9.4 months, but the 10-month lifetime returns only $127.50, leaving about $7.50 of profit per customer). Show the arithmetic (paid CAC $60,000 ÷ 500; payback $120 ÷ $12.75; lifetime margin 10 × $12.75 vs $120). Name what you rejected (the $80 blended CAC and the payback-only “it passes” reading) and why. Name the one thing that would change your mind (a longer customer lifetime or a higher monthly margin, so each customer is worth clearly more than they cost). Keep it to a single page a founder grasps in two minutes. This memo is your own argued claim — not a credential.

11. Sources

Meridian Analytics and Larkspur, and every figure attached to them, are composite — constructed for clean teaching arithmetic, not drawn from or claimed about any real company. The concept definitions used to reason about them are standard; references below.

Concept / claim	Source (publisher)	URL	Accessed
Customer acquisition cost = spend ÷ new customers	Wikipedia — Customer acquisition cost	https://en.wikipedia.org/wiki/Customer_acquisition_cost	2026-07-19
Gross margin = price − cost of service, as a percent	Wikipedia — Gross margin	https://en.wikipedia.org/wiki/Gross_margin	2026-07-19
Payback period = cost ÷ periodic return	Wikipedia — Payback period	https://en.wikipedia.org/wiki/Payback_period	2026-07-19
Customer lifetime value and why lifetime bounds payback	Wikipedia — Customer lifetime value	https://en.wikipedia.org/wiki/Customer_lifetime_value	2026-07-19
Churn rate — customers lost per period	Wikipedia — Churn rate	https://en.wikipedia.org/wiki/Churn_rate	2026-07-19

Next up

Finished this call? Continue the Marketing track:

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