In Mishnah Chet (eight) of this Perek (chapter), we learned that if even a small amount of Yayin Nesech (prohibited wine) falls into a large amount of permitted wine, that small amount would prohibit the entire mixture. Here, we'll also bring down the view of another Tanna (sage), who says there's still a way to benefit.
The Mishnah says, “Yayin Nesech falls into a giant vat of permitted wine. The entire mixture becomes prohibited for benefit.” Rabban Shimon ben Gamliel says, “There's a way to gain benefit from this wine. You don't have to get rid of all of it. There's a way to actually fix the situation. The entire mixture is allowed to be sold, but you have to subtract the value of the Nesech wine that was poured in.” You're receiving money only for the amount of wine that was permitted. You'd have to deduct the rest of the prohibited wine from that calculation.
So let's say you had $100 worth of permitted wine, and $2 of prohibited wine fell in. So now there's more liquid there. When you're selling the $102 worth of wine, you wouldn't be able to sell it for $102. You'd only be able to sell it for $100, receiving benefit only from whatever was permitted before the mixture took place.