Subaru Outback 2004 ABS, Airbag, and Brake lights are on salvage car.?

Sounds like a control module to me. As long as your airbag lights is on your air bags will NOT deploy in an accident. The ABS, airbag and brakes should all be tied in together with the module. This dealership is trying to rip you off. $550 for "investigation" is crazy! It is true your seatbelts have sensors in them that if the airbag was deployed in the previous crash (you should be able to buy used) they can cause the airbag issue. Did you buy this car from another dealer? I would take it back and have them fix it. Also, make sure your airbags were replaced and not just a cover put over the old deployed bags.