CAPA sell them for $1975 but that is just for the center. Add shipping, a diff bearing kit and fitting at your local diff shop you'd be looking at 3 grand. Most people here seem to think they KAAZ is old hat but I have driven and raced cars with both the trutrak and the KAAZ and I would take the KAAZ any day of the week. It is the bees knees. They can be noisy though but a small amount of friction modifier calms that down. I wouldn't know I had one. Quiet as.
If you were to go down replacing the diff center then I'd suggest spending a little more and getting some lower ratio diff gears put in, they're a great mod and pretty much only cost you parts if you do the whole lot at the same time.
If however you are on a budget, a reshimmed holden LSD is better than nothing.
I hope this gives you a good understanding of available options.