The simplest way to place a cash value on a car is to look at the recently-sold market, and deduct the cost of getting the subject car to that level.
Might want to round that projected repair cost up at least 10% - maybe 20% - due to surprises, price fluctuations, whatever.
That's based on getting a very good look at the car, plus experience.
Appealing combinations and/or sentimentality may add value; mysteries or an overall funkiness generally don't.
TV Auction prices are often inflated well over the actual market, of course.
The vision for what you want the car to be is quite significant, as well.
The appreciation curve is still pretty strong for many old cars, fortunately.