Years ago I said at least $50 for non-exclusive and at least $500 for exclusive.
Times change and I believe it should be higher today, but it just depends on the beat and what you think you're worth, plus what the market is like. Some might charge $5000 but that's probably more for those with connections. I think the majority of sales come through marketing and networking, not so much on the beat selling websites.
The main issue with the beat selling sites is it's been over-saturated for a long time now. Once everyone saw they could sell beats, a bunch of kids in their rooms started pumping out generic beats with Garageband and FL Studio. (No disrespect to Garageband and FL Studio.)
Twitter was a great place for networking like that, but now that site is in the trash.