Nice, I like the idea. Only thing is I think it might be better if after you died, it just kept the score. Since one would have to survive a really long time and play really well just to get even the cheapest upgrade. When it doesn't keep the score like it does now, it has a feeling of just starting everything over. It seems to conflict a little bit with the replayability idea of the upgrades. If the score was just kept between runs, maybe it would be a little better.
Other than that I thought the mechanics and the way the enemies worked, and the different things that happened when you plaid were really good!