Trying to answer my own question... in v3 the parent pays 0 while the child pays the price of two slots. The opportunity cost for the miner to replace slot 10 with the 0-fee parent is simply the fee of the transaction that would otherwise be in slot 10.

So as you point out, they could charge out of bound anything between about 50% and 100% of the child fee. Slightly less because of the slope in fee rate, that effect is tiny.