I think a bigger version of Mine is a no-brainer for Prosperity. Works especially well with treasures that cost 5, and 4 in the case of a Platinum game, which thankfully there's plenty of in Prosperity. The last part is mostly to make sure it's better than Mine on boards with no treasures of interesting values, but I didn't want to make it too strong, as to incentivize players to use the full boost when able.
Unfortunately I can't properly playtest this as I don't have access to Prosperity, and Platinum games is an important part of the card, so balance might be off. After all, Mine is pretty weak.
Edit notes: (also edited the above text slightly to be more accurate with the updated version of the card)
- Also works well with cost-reducers(turning coppers into gold) now when it's a 5 boost rather than 4.
- The last part is now a pretty significant part of the card, but feels very fitting with the money theme of Prosperity.
- The second gained card goes to you discard pile, should this be written out? If so, is "(to discard)" well formulated enough or do you have to write something longer?
- Could replace Mint if that leaves, both has to do with trashing and gaining treasures.
Old version: