We have a saying in our shop that we use to describe what almost all people who are unable to do their own work see in a "restored" car. We say "It's not what it is, it's what it looks like". This explains the many "restored" cars we see that do not run right or do not run at all. We are presently doing chassis rebuilding on two cars that were brought to us. Both are beautiful and expensive (over $100.000) "restorations" but neither ran well enough to be used. Now we are expected to rebuild the chassis on these without messing up the "what you see" part of the restoration. It was explained to both owners that we would only do the best we could do to save what was done by others, they would be responsible for what had to be redone. Not restoring the chassis is not restoring the car, it's just fixing it up. We do not do that kind of work, and tell those that ask for that kind of work to take it elsewhere.
Generally in this shop a complete restoration to new will cost the owner just about what John S above said, roughly $100,000 t0 $200,000, and can be considerably more depending upon what we have to start with, and sometimes that's not a lot. What I just printed is what I tell those who need a "guesstimate", on a job. I might add we do not lack for work.
Good luck with the GTO!