I'd go with waynovetten' s advice, I've done a few gearboxes and none have been cheap to fix. I usually end up buying one from a scrap yard or ebay and using it as a donor taking the replacement parts I need from it and building a new box. If your gearbox stopped quickley then damage could be limited and much of it useable, check the cases carefully around the bearings and pin locations. Drop the engine oil from the gearbox and see if anything falls out. Take the clutch cover off, a simple job and may provide info on how bad it is.