Wow, you guys really have low scores.
I liked how you could upgrade by defeating the bosses, but you need to add something like a menu to add specific upgrades (such as more health, faster rate of fire and damage) that can be gained by spending your score. Sure, it might make people a little overpowered with the building powerups, but it also makes it a little more challenging later, as you have to decide: Do I want to fire faster now, or save my score so people can see how 1337 I am.
My end score was 273192, for those people who are trying to boast their egos by score. :P