CAC is the fully loaded cost of acquiring a paying client, including media, agency fees, tooling and sales time.
CAC is broader than CPL. Where CPL stops at lead capture, CAC extends through the entire acquisition funnel to the moment a prospect becomes a paying client.
Formula: CAC = (Media + Agency fees + Sales cost) / New clients
A firm may have a healthy £45 CPL but a £1,200 CAC once only 4% of leads convert to clients. Tracking both side-by-side is essential to avoid over-optimising on surface metrics.