The GT-V model came with an LSD and most of the GT-t bits. (brakes, suspension etc)
If yours is a regular 25GT I assume you're going to have an open diff. Jack the rear of the car up and spin one of the wheels and just observe which way the other side spins. If it spins in the same direction as the one you're spinning, it's an LSD. Well if not, it's obviously open.
And so in that case if it IS open, you should expect to pay somewhere around the $200-400 mark for an R33 ABS diff. The 25GT uses all R33 suspension and diff parts, not sure why.
Not really sure what labour would cost you as it's something most people do at home if it's a direct swap but if you're not confident in doing it yourself maybe someone else has some info to share on who did it for them.
I would highly suggest shimming the diff to get it a bit tighter before it goes in. Very cheap and effective.