They make only what their contract states they should make. Dana White controls a lot of the negotiating between fighters.
Here is an example of the contrast that can be scene between fighter salaries.
In UFC 76 Jardine vs Liddell, Kieth Jardine only earns $14K as Chuck Liddell earned $500,000K
Here are the salaries from that UFC.
Keith Jardine ($14,000) def. Chuck Liddell ($500,000)
Forrest Griffin ($44,000) def. Mauricio "Shogun" Rua ($150,000)
Jon Fitch ($44,000) def. Diego Sanchez ($22,000)
Lyoto Machida ($50,000) def. Kazuhiro Nakamura ($20,000)
Tyson Griffin ($24,000) def. Thiago Tavares ($9,000)
Rich Clementi ($24,000) def. Anthony Johnson ($5,000)
Jeremy Stephens ($6,000) def. Diego Saraiva ($3,000)
Christian Wellisch ($14,000) def. Scott Junk ($4,000)
Matt Wiman ($16,000) def. Michihiro Omigawa ($5,000)