According to the dev, "As you know, all lifeless, severed barbarian heads are only worth about 600 MS points on the black market, so that’s how much we’ll be charging for a copy of Castle Crashers from now through Monday, May 21st. Not a bad deal when you consider you’re getting the FULL game… that’s much better than only a portion of a barbarian corpse."
The sale is somewhat of a surprise as it was not mentioned in Major Nelson's weekly content update.