I like the idea, but it doesn't really seem like a supply pile that you can really take advantage of; if you can run an engine, for instance, what's the point of picking up multiple copies?
As far as I know, there's no overpay event yet in official or fan cards (could be wrong, I'm not a fan card aficionado), so I think this could be a neat first. 2* seems most balanced to me (possibly 1*), but it would give a big advantage to 4-3 openings over 3-4.
Also, if it was an event, you might be able to take out the copper-clause. Get $4 on a turn and there's nothing good to do, then turning it into 2 coin tokens would have it's niches.