The approx price depends on what all you replace. Most people upgrade their clutch and flywheel, and throw an lsd in at this time cause its the easiest time to. People replace their syncros (sp?) and buy an upgraded shift tower as well. So...anywhere from...$1000 to $3000 if you do the work yourself I think is a fair estimate.
As far as needing to upgrade your tranny with an LSD...is it bsolutely necessary to have to drive the car...no. You do not need to upgrade...but the stock Diff is a very weak part in our cars, and it won't last long at all if you bang on it a couple of times. And while the engine is out and the tranny is out, this is the time you should invest a few more dollars and be sure that you have nothing to worry about. 99.9% of people here wll tell you your stupid if you do not replace your dif with a quaife or torsen...and I agree.
