It's as close as you'll come to a definite figure for the conversion cost. It really depends on a lot of factors. I'm currently doing a gze conversion into an ae92 sx. Not including the cost of the car I'm up to about 5k and I haven't finished yet. I'm predicting it will cost me about 7k all up but I'm pretty much going all out. I also had to buy about 1.5k worth of tools which you won't have to since you're not doing it yourself.
You could probably get away with far less. Here's a rough rundown on what you'll probably need:
Half cut with lsd $2500
EFI fuel tank and lines $150
waterpump $140
Filters and fluids $100
timing belt $50
That's about 3k in parts just for the bare essentials, plus other stuff I've missed. That's if nothing is broken or worn. Plus you'll need a bonnet scoop unless you plan to get a front mount intercooler.
Labour charges really depend on who you take it to. If you want to save money take it to somebody who has done this type of conversion before. I have heard that anthony kellam in brisbane is the man to talk to. His contact details are on the twincam.org forum as well.
Having said all that I think it would just be cheaper to buy one which has already been converted and that you know works, unless you really want a gze powered ae82 sedan.