CPA measures efficiency at the point that matters most: the conversion. Depending on the business, the 'acquisition' might be a purchase, a booked appointment, a demo request, or a signed customer. CPA is closely related to cost per lead, but it's usually defined deeper in the funnel — closer to revenue.
The right CPA is the one your economics can support. A business with high customer lifetime value can afford a much higher CPA than one selling a single low-margin product. Setting the target from margin and lifetime value — rather than copying an industry average — is what keeps scaling profitable.
Example
A campaign that spends $6,000 and produces 40 booked jobs has a CPA of $150 ($6,000 ÷ 40).
How Sloss Digital uses it
We work backward from each client's margins and close rates to set a CPA the business can grow on, then use tools like Target CPA bidding to hold that line as budgets scale.