Whether you'd ask your intended to sign a dotted line or not, the idea of charging per cheat seems counterintuitive — especially if the person has money. It's like parking tickets or late fees or that 30 Rock episode where Tracy decides he can say whatever he wants on TV if he just pays the FCC $50,000 for each offense. Weigh the odds, and decide what you can afford.
Would you really want $100,000 to be the only thing keeping your spouse faithful, though? Then again, cheating is cheating and money is good. Why not just ensure he pays (you) for his crime?