Well, as far as I know not many of them do. The first series, 2 "celebs" went on it, Vicky Pattison and Ricci Guarnaccio. I think they would be the only 2 that were paid. Then on the second, Gaz Beadle and Charlotte Crosby. Then series 4, Scotty T. However the 5th series is an all star series so all of them got paid for coming back, including Bear. I have no idea if they get much for doing it though.
