Pro Tip for Buying Power-Ups
If you buy power ups from most expensive to least expensive you can get all the upgrades for less money!
I still had a few levels to buy for a couple of the cheaper power ups (max health, regen), but ran out of money.
I refunded my powerups and bought them again starting with the most expensive.
The way the price increases work make it cheaper in the end if you do it that way.