I haven't played with this card yet, so take this with a grain of salt, but I feel like people are viewing this card wrong. Other people have discussed the strengths of coin tokens in general, and while this card can generate an obscene amount with the right deck, I think it will be much more useful as a support card, not the payoff card, though it will shine as the payoff every once in a while. Adding a handful of tokens over the course of the game offers an incredible amount of flexibility.
This is of course a card which screams engine. And it gives one crucial engine piece with +buy, though it's a bit pricey. Still, engines usually only need a couple cards with +buy, unlike other vanilla bonuses, so even one or two MG's will be helpful. And of course the coin tokens usefulness is delayed one turn, but with a steady stream of them coming in, it shouldn't be too big of a deal. And in a decent engine, you are usually buying multiple cards per turn anyway, not counting any extra buys you could use on coppers. With 2 MG's buying 3 cards per turn thats 6 coin tokens per turn. And you can do even better than that if you can deal with extra copper buys.
I think if this card cost $4 it would be a no brainer in a large percentage of engines. Pricing it at $5 certainly weakens it a little, but you probably weren't opening with it anyway.