Acquisition cost, lifetime value, and payback period are the three numbers that decide whether your growth engine compounds or quietly burns cash. This section covers how to calculate them honestly, segment them by channel and cohort, and use them to set spend limits, pricing, and hiring plans you can defend in a board meeting. It's built for founders, growth leads, and RevOps teams who are tired of vanity blended metrics and want unit economics that hold up under scrutiny — and actually change what you do next quarter.