as far as rebuild prices go, thats a widely variable question, and it depends what extent you want to go to. Generally when you strip and rebuild a box, you have no idea what its going to require until its in pieces. It may just need synchros, or it could be that you also have excessively worn mainshafts, bearings and clusters, and you'll only find out when its apart.
If you opt for Nissan genuine bearings etc, it may cost quite a lot. Some places will do a changeover box (Ie. one thats already rebuilt in exchange for your box) for a set price..... Personally, I think a cheaper option may well be to buy a secondhand gearbox. You can collect RB25 boxes for around $550, but of course you then need a bellhousing to suit, and may need to do a little cutting of the trans tunnel - not sure about how the splines line up.
But anyway, 3 options really (Well, 4):
- Get your current box rebuilt.
- Go for a changeover box, might be hard to find for a 260/280.
- Go secondhand (Probably cheapest, of course risk is if you can't drive it in car first in a private sale... most wreckers should supply some kind of warranty.)
- Or, practise those double clutches! I can give some hints if you want, its not hard at all, and its pretty satisfying to get it right, even with stuffed synchros the gears will just fall in - in fact, its easier to learn on a stuffed box, cause you know when you've done it right!