They became mandatory in the UK in 2003 I believe. A bit disconcerting that they didn't go off if if was anything like a decent front end whack.
Air bags going off will write off a 25k car never mind a 2.5k car. They basically trash the front of the car when they go off. If the car is worth 2.5k I reckon that if there is quite a bit of damage it will almost certainly be written off. It could cost the insurance company half that amount in hire car fees alone for the time it's waiting for repair.