Just to throw some figures out there:
Anderson Silva first fight against Leben was $18k/$18k. His 2nd fight pushed up to $25k/$25 when he beat Franklin for the title. His last fight was $600k to show and $200k win bonus.
Jon Jones in his 2nd fight was making $7k/$7k. HIs fight against Sonnen made him $400k.
GSP in his first fight made $3k/$3k. NOw he makes $400k show money.
These guys earned it. They kept training, improving their skills, and winning. They became champs, rightfully so with their hard work and dedication.
Any of these undercard fighters have a chance to make that kinda money. Frankie Edgar, Cain, JDS, Jones all started on the undercard. Nobody knew who they were until they started putting winning streaks together. For that, they got those big money contracts.