From what I understand most of them have a certain downside guarantee, a salary floor. They get paid based on A) live events and B)royalties for merch and dvds and such. If at the end of the year that doesn't total up to the downside guarantee, they get a lump sum payment to make up the difference. But they get paid for each show they work, using a formula only Vince knows that pays them based on attendance, for PPV's the buyrate (I think), and where they are on the card/how important Vince thinks they are to the event.
(edited by spf on 29.4.11 1006) 2007 and 2008 W-League Fantasy Football champion!
Starting out, I believe they get a base salary which covers most expenses since WWE doesn't pay for everything mostly the plane trips and some of the hotels. Then you get part of the gate, merchandising and so on. If you do a great Vince gives out bonuses.
One interesting note, Lance Storm once mentioned in a Q&A that his paycheck was always in Canadian dollars so there wasn't any issue with depositing it in his bank account. Not sure if that applies only to Canadians or to anyone who lives in another country. (Do any of the wrestlers from outside of North America actually fly back to their native country on a regular basis or do they maintain residences in the US?)