Very satisfying upgrading system and a strong addictive loop :)
The start is a bit too harsh - it is really hard to get anywhere at first, and it looks like the best strategy after a while is just to wait to get 10+ interest, as it pays so much more than anything else - even on the level where I was shooting down the aliens the interest rate was by far the main source of income. I think shooting things up (which is quite hard) should be much more rewarding.
Overall - great submission! I'd be playing for quite a while, but the game crashed after I reached ~300 meters and killed a few aliens.