All rather depends, in some circumstances they are not in a position to demand a price, they are given one - take it or leave it, these are the bigger labels or 'Sooty' as he can give you a break as a newbie. Where they can demand a price, it's usually for Fred in the Shed. However, they can be also be under contract to a label, particularly with labels such as Bluebird. But they earn more as a pro$$ie, they don't get paid as much as you think unless you are Sasha Grey.
Also the twats at UKAP (UK Adult Producers) have been trying to control the industry for years.
In short, not much more than 'gash for cash'.