The things that are always killers:
1. Body work and paint. Never, ever believe what you are quoted. Triple that number, then multiply the answer by 2. And it still won't be to the quality of work they told you..
2. Nickle and dime pieces.. Again, multiply what you think you'll spend on those little parts by 5. 'Oh, headlight adjusters should be cheap and a good match',etc.. LOL
3. Chrome and brightwork.. need I say more?
4. The 'missing' screw, nut, bolt, fastener....
So, I would say if you started out with a complete, but 'crusty' (northeast crusty, not pacific west crusty) GTO.. $30,000 + cost of the car for a 'nice' driver.. $50,000 for something 'better', $70000 for a show car.
Now, if you have the time, patience, room, mental fortitude to do much of the work yourself (strip, bag and tag the car, restore/ rebuild the rolling frame, restore the interior, send out the bright work/ polish bright work/ replate bolts/ etc, then assemble the car), you will save a large percentage of those costs.. PLUS have the satisfaction of accomplishment
.