Saw a post a while back on another board where a guy that does professional restorations figured it took about 1,000 hours and his hourly rate was $50.00/hr. So that would be $50,000 in labor. Then add in parts.
If you can do it all yourself and have the needed tools, then you would only have the parts cost and save about $50,000.
Just to add what Tom73 and 69 MyWay posted, if you need to replace frame, used or new, the book shows about 28 hrs. labor. Multiply that by the hourly rate your shop charges and that should be the cost of the job. Now of course the money starts adding up when you start adding the cost of needed parts. The cost of the frame, of course, brake and fuel lines, body mount kit. When they tranfer the drive train and suspension components over to new frame, that's when you want to take care of any other questionable stuff. Trailing arms, front suspension, fuel tank, engine, tranny... It's true that it could cost $10k to $50K, depending what it needs and what you want the car to be when it's done. Sit down whith whoever is goin to do the job, after they go over the car, and try to get an itemized estimate so that there won't be any surprises after they start. I am going to have the frame in my '69 changed out this fall along with some lower birdcage work and it should cost about $8-$10K, (with a used sanblasted and painted frame). I updated front and rear suspension along with a new fuel tank over the last couple of years to save some money when this job is done. Vette Products of Michigan is producing brand new Vette frames at a cost of $3,600 for a C3.