Blended CAC, and why your last-click dashboard disagrees
Every first call includes a version of this exchange. The founder says acquisition costs are up. The media buyer opens the ads manager and shows a ROAS that looks fine. Both are right, because they are reading different fictions.
What last-click actually measures
Your ads platform reports conversions it can claim. That number drifts further from reality every year: modeled conversions, view-through windows, double-counting across platforms. Add up the "revenue" claimed by Meta, Google and TikTok in a typical account and you get 130–180% of actual store revenue.
Last-click in your analytics tool has the opposite bias — it under-credits anything that works early in the journey and hands the win to branded search and direct.
Neither number is useless. Both are wrong in stable, exploitable ways.
The one number we report
Blended CAC: all paid spend in a period, divided by all new customers in that period. No attribution model, no platform claims, nothing to argue about.
Blended CAC = total ad spend / new customers acquired
Two details matter:
- New customers only. Retargeting your existing customers into the denominator flatters the number and hides the real cost of growth.
- Contribution, not revenue. We pair blended CAC with contribution margin per first order. A falling CAC that is bought with discounts is not an improvement.
How to move budget with it
Platform numbers are still useful — as relative signals inside a channel. The weekly loop we run:
- Blended CAC and contribution margin set the total budget: if last week's marginal spend cleared the margin bar, budget goes up; if not, down.
- Platform-reported CPA decides where inside each channel the money goes — it is biased, but consistently biased, so week-over-week deltas are real information.
- Every quarter, a crude incrementality check on the biggest line item: geo holdout or a two-week pause on a retargeting layer. The first time a client runs this, the retargeting budget usually shrinks.
Why dashboards disagree, permanently
There is no configuration in which the ads manager, analytics and your bank statement agree. Modeled attribution guarantees it. The mistake is not the disagreement — it is letting each team quote whichever number makes their week look good.
Pick blended CAC as the number that decides money, keep platform metrics as steering signals, and write that down where everyone can see it.
If you want a second pair of eyes on the gap between your dashboard and your bank statement, that is a teardown — the measurement readout is page one.