About $600-900 if you take it to them bare depending on how much stuff they need to replace inside. Gearbox bearings aren't cheap anyway.
Though it may be even cheaper to just take them the shafts. If you are at all mechanically minded it is simple to split the box and remove the two/three shafts. Rejoining the two halves of the input shaft is the only tricky part.
Actually stripping all the bearings and gears off is harder. I did it, but there was no way it was going back together once I'd finished. I think there are some special tools you need to remove various circlips, I used a hammer and screwdriver...