Sample frameDay 288·Growth · Acquire

Morning. Your cost per customer is an average, and the decision in front of you isn't.

Growth.

The Marginal Customer

5 min read·Apply by lunch

The question

Of the customers you won last month, how many arrived through a channel you actually paid for — and of the money you spent winning them, did the last slice buy customers at the same price as the first? The gap between those two answers is the size of the decision you've been making blind.

The idea

Day 131 put you in front of Bill Gurley's attack on the lifetime-value formula, and left one of his objections doing only half its work: acquisition cost isn't a constant. Spend harder and it rises, because you buy the most responsive people first and everyone after them costs more to persuade. There it was a warning about a ratio built at one level of spending and quoted at another. Point the same objection at the acquisition side and it does something else — it splits the single figure most teams quote into three. Blended cost per acquisition divides all your spend by all your customers, which charges nothing for the people who arrived through word of mouth, an old article or a friend. Paid cost per acquisition divides paid spend by the customers that spend actually won, and it's always higher. Marginal cost per acquisition asks something else again: what did the last slice of spend cost you, per customer, in this channel, this month? Only the third answers the question you're really facing, because the question is never what a customer cost on average. It's what the next slice of budget costs you per customer, and that price moves every time the budget does.

The framework

01
Three numbers, and only one of them makes a decision.
Write all three down this morning for the same period. Blended is total spend over total new customers, and it's the one on the dashboard. Paid is paid spend over the customers that spend won, which is usually a good deal higher and is the honest cost of buying growth. Marginal is the awkward one: split a channel's spend into ascending steps, put the customers each step delivered beside it, and read the cost of the last step alone. The blended figure is a scoreboard. The marginal figure is a price, and prices are what decisions get made against.
02
Spend in steps and let the curve show itself.
No channel has one price. It has a curve, and the curve bends upwards because cheap demand is finite — the people already searching your name, already in the audience, already halfway to buying. Raise the budget and the platform has to reach further out, to less responsive people, in order to spend it. You'll see the shape without any special tooling: hold a channel at one budget for a fortnight, step it up, hold again, then compare customers per dollar in the second window against the first. Do that twice and you've got a curve you can plan against.
03
Would that customer have arrived anyway?
Attribution credits a channel for anyone who touched it, which flatters whatever sits closest to a purchase already decided — brand search, retargeting, the discount email to people mid-checkout. Thomas Blake, Chris Nosko and Steven Tadelis, writing in Econometrica in 2015, had eBay stop bidding on its own brand keywords across Yahoo! and MSN while Google carried on as a control. Almost all the paid clicks it lost came straight back through the free link underneath, because those buyers were arriving anyway. The honest test of a channel isn't what the dashboard attributes to it. It's what changes when you switch it off.
04
Equal at the margin, not best on average.
Marginal cost only means anything against a ceiling, and you built that ceiling on Day 127 — it falls out of the payback arithmetic, and it belongs on the page before you brief anything. But a ceiling only tells you when to stop. Where the money goes is a separate rule and a harder one: budgets belong where the marginal returns are equal, not where the averages look best. A channel with a mediocre blended figure and a flat curve is often a better home for the money than a champion already bending — and channels don't announce their saturation. They drift into it while the blended figure stays comfortable.

Members only · 4 principles + template + AI mentor

364 more frameworks are waiting.

The Marginal Customer is Day 1 of 365. One framework every morning for a year — across strategy, sales, negotiation, leadership, and more.

$1 /day

Billed $365/yr · cancel any time

365 frameworks

12 topics · templates · AI mentor

Reserve your place — $365/yr →

One payment a year · the whole library · no auto-renewal.

More Growth frameworks

All 24 growth frameworks →