Does this make sense as an idea? Is it balanceable?
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$? - Event
Once per turn: When you buy this, you may overpay. If you do, choose a non-empty Supply pile costing the amount you overpaid and remove it from the Supply until the start of your next turn.
The idea being that you can pay to make a certain card unavailable for purchase (along with other tricks), but you have to pay the card's cost to do so. For the sake of not making it completely stupid, I would rule that you ignore that pile completely for the purpose of checking game-end conditions, meaning that making the Provinces disappear doesn't end the game immediately. The non-empty pile is mainly so you aren't expected to remember how much a card costs, but it has other effects. I would mostly anticipate this being used to try to control endgame to an extent, especially where PPR or a similar effect is in play or when you think your opponent is about to megaturn and you're only a turn away from your own.