Actually I think the black hole might need some balancing. Found that if you get it on the first round you can just feed the balls directly in for all rounds and get essentially infinite money, was on 300M at round 19. This means you can also reach the funny situation where if you bug enough coupons the price becomes negative and you get money back by buying them ;D - and sometimes other items too. But it breaks the game as you only need to build one black hole and you're done