Maybe something has changed with Randi's site, but the videos and bits I've seen on the challenge are that the "contestant" sets the parameters of the test. Even if Randi's group requires the initial input from the contestant to be modified, the contestant still agrees to the parameters of the test and is supposed to be confident they'll win the prize. If someone was a credible contestant, they'd actually have the abilities they claim to have, and very few agreeable parameters would probably impede their million dollar payday. It wouldn't matter if Randi was credible or not, the contestant would simply exhibit their abilities and satisfy the requirements. If Randi didn't pay, there'd be a huge, very public lawsuit to take down the million dollar skeptic.