The wreckers is your best bet if you're a penny pincher. But even then there are no guarantees as to the condition of the glass (unless you see it).
Don't sneer at the price, even from BMW. Once you see the glass itself, you'll be surprised to see how complex it actually is. It's not just a piece of glass as most think it is. It has built in heating elements and electrical connections along with this fluid that enables the auto-dimming. Pretty intricate piece.