The thing that I don't buy with this Brock Lesnar conspiracy theory is that Brock is a prideful person, he likes to be the best at what he does, otherwise he would probably still be in the WWE if he wanted easy money.
Brock's bank account must be stacked also, he probably doesn't need one last final payday from the UFC. He and his Wife both worked for the WWE, both made millions of dollars. They live a laid back lifestyle in the country, so they probably don't have too many huge bills to pay like other athletes/millionaires.
Brock Lesnar also started working out for this fight, he visited Randy Couture and trained with him for a while. That would be going to a lot of trouble and unnecessary workouts to make this plan. He wouldn't need to be training at all if this was really how it all worked out.
I think Brock likes money more than he likes his pride. Sorry, but someone doing professional wrestling in a time where we have MMA doesn't strike me as prideful, especially considering he was a college wrestler. College wrestlers went one of three places: MMA, WWE or a desk job. If he wanted to fulfill his pride and competitive spirit he would have went with MMA. If he cared about his pride he wouldn't have been fake fighting with other greased up men in speedos to soap opera plots. Brock may be a good person to his family and friends and all of that, but I think he's far from a sellout.