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The first thing that they will have to do is run a scan on the air bag system to make sure that nothing is still triggering the system. If there is nothing that is still active in the air bag system, then they will simply replace the air bag system.
Now if you choose to purchase the air bag from the dealer and have them install it, expect to pay around $800-$1000. This is only if they need to just replace the air bag. You can get this done cheaper if you buy the air bag from somewhere other than dealer ( from another car, used, salvage yard).