Just depends what they've done and what their charging rate is. Transmission out couple of hours then any stripping they have done. Then say three hours to stick new bits back in. Plus cost of new units and oils consumables. Times may seem tight to some, but if they are specialists they should have the tooling to make them easily possible.
if your symptoms are black oil no forward drive ,drive in reverse but slips most likely just gearbox ,now it is possible for anything however remote to happen but id ask for more info
They said around £1250 for recon gearbox and transfer fitted not sure if was extra for them to take the old one off and have a look should of asked really