It all depends on what needs replacing. When my box was done recently it needed the whole 3/4 and 5/6 synchro hub replaced which were expensive as parts compared to just the synchros.
Labour is usually expensive for a gearbox rebuild as it takes a while to remove it from the car.
The last gearbox I paid someone to rebuild for me was about £400 ish. It that was delivering the gearbox to them already removed from the car.
I had a few gearbox rebuilds done last year.
If you do take the box out replace the slave cylinder whilst you are there. Slaves are cheap compared to them failing and needing a gearbox rebuild.
Sycnhros wear and there is not much you can do to stop it. If you want to keep a reconditioned gearbox in good condition then change the gearbox oil regularly and be patient when changing gears.
