Blended CPA

Blended CPA is total paid spend divided by total deduplicated conversions, across every channel at once. Channel CPAs rely on each platform's self-attribution, and platforms grade their own homework. When the channel numbers look better than the blend, the blend is telling the truth.

Smart Bidding and its equivalents set bids per auction against the platform's own conversion predictions; you steer with a target rather than keyword bids. That automation optimizes each platform's attributed conversions, which is exactly the number the platform also reports to you. Two platforms can both claim the same customer, both hit their targets, and both look efficient while the company's real acquisition cost rises. The blend is immune to double-counting by construction: one numerator (all spend), one denominator (real customers). Reconciliation with finance's version is on CPA vs CAC.

• Halo: prospecting on one channel lifts branded search on another. Channel CPA books the value to the closer, not the opener; the blend prices the system.
• Cannibalization: paid clicks that would have converted organically inflate every platform CPA's apparent efficiency. Blend against total new customers, not just paid-attributed ones, to see it.
• Mix shift: the blend rises when spend moves from cheap retargeting into expensive prospecting even if every channel improved. Read the blend beside the mix, not instead of it.

Worked: $8,000 at 160 conversions ($50) plus $4,000 at 50 ($80) blends to $12,000 ÷ 210 = $57.14. The red channel drags the blend; whether to cut it depends on whether its conversions are incremental, which channel attribution cannot tell you. The arithmetic is on the formula page; the target you steer automation with belongs on Target CPA.