would you have felt better if instead of them having a fixed price for labour to strip and rebuild a box they had two prices 280 to strip and repair 380 to strip and repair and fit new parts.
Most gearbox companies have a similar price for the mt75 when it come to labour charges.Quaife charge 300 labour plus carriage plus vat to fit their 3k gear kit, we have a similar price for that. seems fair to me to charge that labour fee to strip check and rebuild your box.
the suggestion of 3 k to fit all new standard parts was not the correct advice on an mt75,unless of course the owner wanted all new standard parts and not a straight cut.

The standard mt75 gearbox is not any good for any more than standard bhp