I have only limited aluminum casting experience (research work), but from observing commercial practises, I think you will end up paying one way or another. Either a direct charge as you report, or it is built into their overhead and pricing.
However, some foundaries run 'cleaner' than others (reduced dross formation) by protective gas shielding, use of master alloys & faster analyses for less holding time at temperature while achieving alloy composition, etc.
There may not be a market for the dross; it can only be recovered by the few primary producers who electrolytically smelt purified alumina. The alloying elements Si, Fe, etc., are certainly undesirable in primary 99% ingot. Secondary or alloyed 90% ingot, as from recycled metal, normally sells at a lower price, by $20-$90/ton.