Is this to set a value for insurance? Or just to sell?
If it is for insurance, Hagerty is a great company. They will basically let you set a "reasonable " value and then if stolen or destroyed (fire) you get the full value you asked for. Obviously you cannot set a value for a 19770 Pinto with no engine at $50k and get it. But for a nice GT/CS you should be able to insure it in a range say $20k to $50k. Monthly premiums on $20k are far less than $50k.
Sometimes on a full restoration you have more invested than the "going" value of the car, but if it is not to far our of range you can pay for that amount of coverage.
Hope that makes sense?
Rob