Reading marginal CAC: where spend stops being incremental
Blended CAC hides the most important number in your media plan — the cost of the next dollar of spend. Here's how to find the financeable band.
Blended CAC averages your cheapest and most expensive customers together. It is comforting and almost useless for deciding how much more to spend. The number that governs scaling is marginal CAC: the cost to acquire the next incremental customer.
The rising curve
As you scale a channel, the marginal cost of each additional customer rises. At some point the next dollar of spend buys a customer whose lifetime gross profit no longer covers acquisition. That point — not your blended average — is the ceiling on profitable, financeable growth.
Measuring it honestly
Geo-holdouts, matched-market tests, and conversion-lift experiments reveal true incremental CAC versus platform-attributed CAC. Where experiments aren't feasible, calibrated media-mix and causal models estimate the curve — but they should always be checked against real experiment results.
We deploy capital into the spend band that remains incremental and profitable, and keep it out of over-saturated media. Financing the wrong part of the curve helps no one.
Educational content only — not an offer, commitment to fund, or financial advice. Figures are illustrative.