Paper Currency
Treasure
Cost: $4
Value: $3
When you play Paper Currency, the player to your left reveals his hand. For each copy of Paper Currency in his hand, reduce the value of this card by $1, to a minimum of zero.
I don't think I have the wording in proper formal language, but hopefully you get the idea. It's worth less the more of it there is in play.
Doing it with just the player to your left means that you could get an unfair advantage in a multi-player game, where one opponent avoids Paper Currency and thus major advantages a player to his right. You could do it with an "all opponents" deal, but then Paper Currency's value diminishes in larger games.
I doubt it's a very good or well-implemented card right now, but I thought the conceptual mechanic was neat, and was wondering if anyone else had a good way of fixing it.