Basically a combination of what they are worth to you and what you can get for them. Really, beyond that, nobody can tell you.

If you want to hustle beats, make money, you have to sell low though. Even if you don't feel that's what it's worth. Just from dealing with local mc's, they may only buy 1-2 beats for their albums, if that. Nobody can afford to invest $800 on an album if they aren't selling. Add CD duplication to that, their own investments, etc, etc, etc, then they will be getting around $3 profit per album if it's around 1000 copies, assuming they sell every one. So keep that in mind when you start to sell.