To earn the achievement that's called "You Did What?!", you have to earn $50,000.
Cash is earned by defeating enemies or hitting objects that sometimes drop cash as well. You'll also come across tapeworms that, when punched or kicked repeatedly, will also drop tons of cash as well. In fact, if you want earn money quicker, play through level 10 on any of the three difficulties and make your way to the three tapeworms. Punch or kick them all to get some cash, then get the key and open up one of the three treasures to get more cash, preferably the second one, which drops a lot of money. Then defeat Skullmaggedon/Evil Marian and enter the space level. Once you arrive, go all the way to the left, until you can go no further. Then go to the bottom-left corner of the screen and hit down. You should be going down the ladder and when you drop to the bottom, you'll see three treasure chests with two keys. Open them up for more gold and other sorts of goodies. Then exit the chapter and enter it. Rinse, wash, and repeat until you earn $50,000 total or a little over it and the achievement unlocks for good.
Have fun and good luck!