That's the dilemma for elite MMA fighters: security vs take a risk for really big success. If you're good enough to be a dominant star in a lesser org, you'll make pretty good money, reliably. With the UFC, there is a potential for riches (or at least near-riches), but a number of things could go awry. Injury and losing a couple of fights you were expected to win are obvious ones.
It's very rare that the UFC will offer someone big money for their first contract. I assume that's what happened with Brock, but he was a megastar. There must be a couple of better examples, but I can't think of any names at the moment.