I am at Level 17 and have 7 donuts. I have spent a handful by accident, other than that have been diligent in keeping them. The Mystery Box has given me 500 credits every time (about 8 times at least now).

It seems there are some critical components to this game requiring donuts, but no foreseeable way of ever acquiring them without paying cash.

In my mind, this hardly qualifies it as a "free" game.

So, am I missing something, or is there no way of finishing this game without spending well over $100?

The items that you can buy with donuts are 'premium' items and are optional. You don't have to have them to play through the game. It would be nice to be able to acquire donuts without paying through the nose for them. I'd like the volcano, duff beer brewery, etc but they are way overpriced.