that also works. So the answers so far:

-poor house

-count+fairgrounds

-P-stone

-loan

And my initial idea, which was:

-a draw to X card. let's say you have in hand, Necropolis, counting house, watchtower, gold, silver. Your discard pile has 2 coppers. You want to buy a province. You've been tracking your deck and know that the only money left is silver. If you pick up one copper, you need to draw one silver in 3 cards. If you pick up two, you need 1 silver in 2 cards. If you pick up none, you need 2 silver in 4 cards. Picking up exactly one copper gives you the best chance of buying a province.

That was a lot more convulted than I thought it would be. Your solutions are simpler