Right now, its what the other guy is willing pay. because people are using btc as a commodity like gold, because it shares two very important traits of a commodity 1. You can't easily fake it and 2. its rare, only 21million, 1/2 is out and other 1/2 will be mined at a exponentially slow pace, which is think is silly and a passing fad. It will stabilize once people start using it as a currency which is what you use buy a precious commodity and hoard it.