Damaged final drives, gears, synchro' hubs, forks ie those parts which are not usually replaced, will incur a charge representative to the items replacement value, either new or second hand. Additionally the case must be serviceable, ie. There is no excessive wear on the idler bearing surface, all threads must be intact and sound, including sump plug, and no cracks in the casing. A minor level of damage to the exterior of the case is expected, as long as is does not affect the structural integrity.
Helicoils can be used to repair damaged threads in non-critical locations, however, these will be charged for ( £10 per 5/16 or 3/8 UNC coil, £25 for Sump plug )
When you have purchased a gearbox you have the option of sending your old unit back as a trade in. Your gearbox will be assessed using the same criteria as above and a representative grading applied. The grading will then determine the value of the unit.
The linked PDF will explain the grading and values for trade in units...