Use the goblin challenge on hard to get some cold hard cash. It's easy to get over 600k per match. Sure it takes awhile, but after killing those black goblins with meteor you can just let it roll
glitch-ridden. My fully upgraded frost tower will sell for 375$ and at the mid of the game (wave 25) it lost 2000dam from a bit over 3000 to 1200 (now it's back to 5300)
I thought this was tougher than the previous on my first challenge/fight. After that I've come to the conclusion that it's even easier than the previous :(