Price is decided by supply and demand, you can't set a price for bitcoins and hope it will be fixed forever.
True, but if there is something like Bitcoin central bank, their goal can be 1500 USD, so buy until price reach 1500 USD, and sell if over 1500 USD. This way price can fixed at 1500 USD, as long as the central bank has enought USD and Bitcoins to feed the market