I had same vehicle, bought from auction unfortunately, most places qouted around �400 for new gearbox!!! If the problem is with one particular gear, some garages will take the box apart and replace just that gear.
Personally, I sold the car...sorry!
Justin
Get a second hand box fitted - not worth spending a lot of money on a car of this age. You should be able to get a box for about �60 about same to get mechanic to fit as a private job. It may be wise to have a clutch assembly fitted while the box is out.