You don't have to buy it in the endgame. You can buy it *instead* of provinces and be immune to greening. Chapel Big Money basically becomes the Golden Deck without Bishop. Cute, though.
This is a very good point. Now I wonder if it would be interesting to do it with Duchies instead. Maybe something like this:
Aristocrat$6 - Action
+1 Card
+1 Action
You may trash this card. If you do, gain a card costing up to $6.
So the main purpose of it would be to pay a $1 premium on a Duchy that doesn't clog your deck, but that you have to redeem later on. It's like a Great Hall that you have to trigger. However, because of its cost, it has other uses as well. It combos a bit with cost reducers (e.g. you would prefer having this than a green card in a Highway deck, and this can gain Province with enough price reduction). You can also use it to gain other alt VP (Fairgrounds being the big one). It might also be interesting for 3-pile scenarios with Duchy, particularly with Duchess or Border Village. Rush this card and drain the pile, then immediately drain Duchy+Duchess/BV.
I actually think this sounds really cool. Is it workable? Broken?