[sperry]

+1 on what Cory said. If you just suddenly stripped a gear and immediately stopped driving the car around, then you're probably cool for replacing just the busted parts. But if the car was "iffy" for a while before the gear finally went, or if you drove it w/ the damage, I'd be worried about the longevity of the gearbox's bearings and other bits over time.

It all boils down to how long you plan on keeping the car... if it's gotta last another 150k miles, you'll probably want to bite the bullet and rebuild it completely, but if you're only looking for 30k out of it, do it the cheap way and then it's someone else's problem... which isn't cool and reduces the selling price of the car, but hell, times are friggen tough these days and rebuilding gearboxes is expensive.

You really won't know how bad things are until you can get the case cracked open and have someone that knows their shit look at everything. A possible option is to buy a second WRX tranny w/ a different busted gear on the super-cheap, then just pick all the best parts from both boxes to rebuild you one working box... but that's only cheap if you can do the work yourself since labor tends to be the most expensive part of transmission repair.