I just ended up losing a half hour of my life to this game because it was so addictive!! I only wish it was easier to keep gaining more and more currency when you start getting into the 10,000-cost items. With the next upgrade costing 51,200, it took me two full cycles from circles to hexagons to be able to afford it. I'll probably come back and try a different upgrade path to see if i can clear out all the enemies when they're circles!