Thanks guys. I just "wasted" an hour with this thread. I only wish I could get an IM to play me a rook down for some serious cash. Durr is, as Curtain says, about a 950 USCF player. Only he didn't know that!
My estimate of how good a player has to be to have a +EV against an IM a rook up is around 1300. Against a GM maybe 1500.
The difference between a normal IM and a GM in this bet is almost zero when facing low rated players. This should be extremely obvious and if you don't realize this you shouldn't be so cocky about how easily you'd win.