Nicely designed game, but i noticed there where a couple bugs. as mentioned below, prices where inconsistent with the price listed. In addition, at the end of a level when my health was replenished, i could be hit by missles and take damage, forcing me o start the next wave with half health.
I liked the graphics and overall look of the game, but maybe a change of environment or interactive surroundings would make the game seem less repetitive.
Other then that, good job and keep up the nice work.