Seriously, once you enter a price into the appropriate checkout field, you buy the game for that amount. We'd encourage you to cough up as much as you can spare though, because 2D Boy totally deserves your money. The sale ends Monday, October 19, so start digging under those couch cushions.
You may also want to set aside some loose change for the upcoming World of Goo iPhone port. 2D Boy has a build running on a 3GS right now and is working to get the game going on the vanilla 3G. We'll be sure to keep you posted on its progress.