Now that we’ve set our base bid and calculated RPC for each placement, we can compute the final modifiers. The formula we use is:
Modifier = (((RPC × Target ACOS) ÷ Base Bid) - 1) x 100
This tells us how much we can increase the bid for each placement while staying within our ACOS target. A higher RPC means that placement is generating more revenue per click — and therefore can afford a higher bid.
Negative modifiers are ignored since Amazon doesn’t support them; instead, poor placements are automatically throttled by the low base bid. For this example here are the inputs with an example calculation:
> New Base Bid from step 1: $2.22
> Target ACOS: 35%
> Modifier: ((RPC × Target ACOS) ÷ Base Bid) - 1 ->> ((RPC × 0.35) ÷ 2.22) - 1