Well, the mole won't go "undiscovered" or "discovered", rather, the winner is the one who knows the most about who the mole is. So the winner of the game might get 7/10 questions about the mole right, so they wouldn't be able to determine whether or not the mole was "detected" or not. Chances are, with only 3 people left, the contestants may both know by now which is the mole (or, it's possible that neither of them do... they have a 50/50 chance at this point). Last time by this point I know they all knew (they said later), it was just a question of who did better on the mole quiz to see who was the winner.
Anyway, my point is that I would think the mole is paid to do their job regardless of how well they "trick" the other players, and I would guess that they are paid some pre-determined amount of money, just as the host is paid for his role. It's like an acting job.
And, of course, the winner of the mole (the contestant) gets the pot.
Hope that made a little sense at least, but I guess to answer your question I don't know how much money the mole gets, LOL.