They are indeed expensive to replace, an accident that deploys most airbags will mean a heavily damaged car externally, therefore leading to expensive internal 'damage'. Depending upon the point of impact they will not all deploy, this will depend upon the age of the car and the level of smartness of the controller. Head-ons will deploy drivers and passengers and leave the thorax and curtains intact.
When I was designing the assembly and test machines for MG/Rover, Freelander, Transits, Toyota's etc a drivers unit was typically £1000! They are dangerous things when out of a car as the generator is classified as an incendiary device and needs to be housed in a certified cage until fitted.
Suggest you get someone to plug in a code reader to see which one is faulty. Are the seats fitted with thorax airbags? If not, are the seats as fitted from the factory? If someone has fitted non standard seats then the originals may have had thorax airbags and the loose plug is 'looking' for the module. You can fool this by inserting a resistor across the plug terminals.