Cost Per Acquisition (CPA)
A single channel's CPA is not the company's CAC. Confusing the two leads to judging an ad channel by the same yardstick as the entire commercial engine, a scale error.
“An excellent CPA on a channel acquiring customers who churn by month three isn't efficiency. It's a retention problem disguised as a marketing win.”
Otto GTM ObservatoryCPA per channel, CAC per company: two different questions
CPA answers a tactical question: how much does an acquisition through this specific channel cost, considering only direct media spend. CAC answers a strategic question: how much does it overall cost to acquire a customer, including sales salaries, tools, content, and overhead that no single channel can claim alone.
Using channel CPA as a proxy for company CAC systematically leads to underestimating the true acquisition cost, because it ignores the entire support structure that makes that conversion possible. CPA is useful for optimizing the channel mix, not for evaluating the company's overall unit economics.
Anti-patterns
Common mistake: presenting the cheapest channel's CPA to the board as if it represented the company's overall CAC, generating unrealistic unit economics expectations relative to real costs.
Second mistake: choosing the channel with the lowest CPA without verifying the LTV of customers acquired through that channel, optimizing for a low acquisition cost that hides structurally lower customer quality.
Practical Application
A company calculates a €210 CPA on the paid search channel, much lower than the company's overall CAC of €3,400 (which includes sales costs and overhead). By mistakenly presenting CPA as a CAC proxy, the board approves a growth plan based on unit economics 16 times more favorable than reality, an error discovered only at the first quarterly cash review.