MMA (UFC) fighters in general are paid on a per fight basis. Typically they receive a base pay to show and pay to win. The win amount is usually the same as the base amount. For example, if a fighters contract calls for him/her to make $10,000 to show, they would make an additional $10,000 if they win, thus making their pay for that fight $20,000.
The amount varies greatly depending on who the fighter is. Top name fighters typically make $100,000 - $500,000 per fight in contracted pay. The lowest tier fighters in the UFC make $5,000 - $10,000 per fight. All other fighters make everywhere in between.
The UFC also awards three bonuses for KO, submission, and fight of the night. This bonus was $75,000 for UFC 83, their most recent event. The highest tier fighters, the ones who put actually sell the tickets and PPV's, such as Chuck Liddell and Tito Ortiz, receive a percentage of the PPV revenues, although the amount is unknown.
The UFC is also been known to give out "locker room" bonuses to their favorite fighters after a strong performance. They are called this because they are given behind the scenes and are kept private. This practice is one of the reasons Randy Couture resigned from the organization. It has brought a lot of scrutiny to the UFC.
These bonuses are said to be in the millions. Sponsorships are also a significant source of a fighters income, although these figures are not disclosed. The amounts are proportionate with the exposure the fighter will get and their popularity.
In short, the biggest names, such as Chuck Liddell, Matt Hughes, Randy Couture, are millionares, where the the lower tier fighters barely make enough to cover all the expenses they are responsible for. In some cases, even fighters appearing in the UFC have normal jobs as well.
The UFC keeps the amount they pay to their fighters are secret as possible. This gives them the advantage in contract negotiations, and keeps the competition from knowing exactly how much a fighter they may be interested in is being paid. It has caused a demand from the MMA community for transparency.
Fighter pay is a very large and controversial topic going on in MMA right now. You can follow this topic and many more at MMAConvert.com