Expect at least $500 USD.
I got mine done the first time for quite a bit cheaper from www.kaazusa.com
I've taken it back there with the tranny out of the charged even less.
The problem & headache of the lsd install is making sure you set the preload correctly.
there are numerous sized shims that you need to have the lsd sit correctly inside the tranny. you will want to get 2 new bearings, there are 2 different bearings for the gts left and right.
the spedo gear is plastic and is prone to breaking when removed, the bearings & speedo require to be pressed on.
if you dont have a spare car its not somthing to attempt unless you absolutely know what you are doing.