CPA (cost per acquisition)
CPA is advertising spend divided by conversions — the cost of one purchase, lead, or signup. Breakeven CPA for ecommerce equals contribution margin per order.
CPA is only meaningful against an allowable number derived from economics: contribution margin for ecommerce, payback-ceiling math for SaaS and lead gen. A $40 CPA is cheap for one business and ruinous for another.
For lead-based businesses, CPA on raw leads is the most gameable number in marketing — cheap leads that never close inflate apparent efficiency while starving the pipeline. Optimize to the deepest event with sufficient volume and value leads by what they become.
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