I am not a professional builder, but restored one Bronco.
I know that I am very happy that I did a total frame off restoration, but, all the money was parts and only labor I paid for was the painting. As others said, curious what that additional $20K actually gets you. With mine I have about $20K in parts, complete resto. So, I guess their $20K must be including labor and additional parts that would be used if the body is off the frame.
And really, what the heck does that mean saving to do frame on, because you can basically get to everything whether the body is on or off the frame. I did not remove my body until I had it ready to go to paint. I restored the body while it remained on the frame. Then I took it off the frame and while it was at paint I did the frame. Body took me several years, with breaks (burn out). The frame took just a few months. Strip it to bare frame, then clean it however (I sand blasted), then prime, paint and put all the suspension back. Then the engine and drive train. Brakes, exhaust, fuel tank, lines, etc. If you know what you are doing you can just move from job to job and get it done. If you are motivated the frame can be done if a few months, working evenings and weekends.
Then body goes back on the frame and you install the wiring harness, dash. Hook up whatever connects after the body is back (clutch linkage, steering column, brake lines, lights, etc).
Do lots of research (as it appears you are doing) so you know how you are spending your money and get the most bang for the buck from a builder.
Remember you are paying these builders for their knowledge that you could gain if you did the resto yourself. But, you won't have it now and they wouldn't have a business.