Wow! I just read this all the way through. So here's what I think. There are two types of GT40s. The orginal cars including the sub set of safar and H&M cars at less ultima value, and everything else. What I believe we are talking about is the everything else.
I will exclude the avengers and similar kit cars because people on this website will generally do so. They are not what at least 90% of the regulars would consider to be replica GT40s.
Then I would divide the everything else group into currently in production kits such as RCR, ERA, Tornado, CAV etc. and the older out of production makers such as the orginal GTD's, KVA's, and even the old Roaring 40's.
Then I would have a look at the car, consider the current cost to buy the kit, if a current builder and add up the cost of the parts on the car, add a value for paint at about 50%-80% of a mid cost paint job but not more than $5K. Lastly add the value of a current licence plate and SB100 in california. I would put this at another 2 as much as 10K for a california current sb100 car.
For the older builders I would try and come up with the orginal cost of the kit. These are generally are much less than the current cars in production. Usually somewhere around 50 - 75% of the newer cars. My GTD for example cost about 35K as a kit. The current RCR for example cost about 45K and you need to buy a gearbox, so add another 5-10 grand and you have a cost for a current kit as an example of about 50-55K.
Then the additional parts are added to these baseline costs and you will arrive at something usually around 45-55K for a "old" car and 65-85K for a "new" car. Tag on the additional value for title and you have a baseline value.
You will note I attach on value to labor.
Please don't attack me for my evaluation, You can disagree, but I 'm not far off on this. There is always exceptions such as death of the owner, going broke, the old lady leaving, etc. These will effect the value but are on a case by case basis.
My guess is about + or - 10% from my baseline will get you a really good deal depending on if you are buying or selling.