I would check their numbers.
It would be helpful if you told us about your Mirage - what year and trim level it is. I'll assume a 2015 Mirage DE.
My initial idea of a passenger airbag sensor would be the occupancy classification sensor. The service manual describes this sensor: "The occupant classification sensor is installed underneath a rail of the passenger seat to detect the load on the seat."
I don't see that this item is available separate from the assembly, and I don't suppose it is the Position Sensor #6921A028. It may have to be purchased along with the seat cushion.
Other passenger side airbag sensors include the impact sensors for front & side. Those are not too expensive either.
If your technician is referring to the airbag itself, then the quoted cost does appear to be in line with the cost of replacing the seat back assembly. However, you did not mention replacing the passenger seat airbag - so this is confusing...
Section 52B of the service manual describes the Supplemental Restraint System, including diagnostic codes. If you have a code scanner available, it might be helpful to know what code is being set in order to better understand what is going on. The service manual also describes an occupancy classification sensor ECU located under the passenger seat. Top Fuel is on to something here.
It seems that there is some information missing. I would ask for more details on exactly what code your technician found and what component it is related to.
Regarding used parts, consider asking your insurer for advice. It sounds like a good alternative, but don't underestimate the influence of liability concerns.