Nice simple idea, well executed. Lots of fun.
Some structure would help, ie levels or labelled attack waves. There's a bug in the listed price for collectables. Also, the mouse cursor gets lost in the carnage making it confusing to see where you're aiming, and ocassionally letting me click outside the game (and maybe click on a link and screw up my game). Just adding a bold and red targeting reticule would fix that.
Good fun though, nice job.