a project is once per game, which is a different design but neither worse nor better. Maybe nonstrictly better. As a project it doesn't seem too crazy for it to cost 0$ and be a question of when rather than if.
You can make a Project usable multiple times per game by having it remove the cube when it's done.
Yeah, even though this hasn't been done before, I think it's easy enough to implement it this way. Modifying a suggestion from earlier in this thread, you could make the Project wording like this (I also prefer a new method of tracking--I don't like the coin tokens on the project because that seems weird to track for each player):
"When you buy this, you may overpay for it. For each $1 you overpaid, put 1 coin token below your coffers mat. At the start of your turn, move one of those tokens onto your Coffers mat. When there are no coin tokens left below your coffers mat, remove your Project cube from this Project."