The idea is good, I liked how your bullets left are also your life points, makes you be more careful when you are getting into the action, but there are two major problems:
1- There should be more feedback to what the user does, at first I had my doubts of what could you shoot and what not, and since the asteroids and the enemies have a lot of life you have the impression that you are not supossed to kill them in the beginning.
2- You should give more money or have a cheaper price for the weapons, and it would be cool to have a high speed button or something like that to refill your ammo quickly, the game takes too long to get the next weapon and keep advancing.