Working airbag alone is worth $800.
http://realoem.com/bmw/showparts.do?...17&hg=32&fg=35
RealOEM gives you a decent idea of how much the parts cost to order from most dealership's parts department. Subtract 30% for used, and 0-20% for condition, and 5-10% for age and you should get a decent sense of what it's worth.