Fair market value is determined by what a willing buyer will pay a willing seller at a given point in time. Having been on the non-dearler side of the buying and selling of WAY too many cars, I have learned that there is a vast gulf between internet asks/offers and actual transactions. The internets have done nothing but complicate and confuse the matter. I generally dismiss internet chatter on values, primarily because the chatters have no skin in the game.
However, as a guy who just bought a GT, I can share my perspective, however flawed it may be. I was much more interested in condition and history than I was in price. Ironically, while I chose a car with only 800 miles on it, I wasn't looking for an investment car. I just wanted a car that I could modify and customize to my tastes, and then go out and drive at 220 mph. I also wanted a car that was as near to factory new as possible. In other words, I wanted a car where the only patina added would be from me, and the only stories told about the car were mine. I'm kinda selfish that way.
I had dealt with Shelby on a couple of previous exotic car transactions, and I knew from experience that he does his homework, pays great attention to detail, and doesn't suffer fools. I flew out to look at several of the cars he had in stock, and chose one that rang my bell. I have NEVER, EVER, EVER purchased a car without looking at in person, no matter how inconvenient and last-minute the trip may be. This is true even if the car is a new car at the originating dealer. When I look at EBAY, it's usually only for entertainment. When it comes down to the deadly serious business of forking over money for the genuine article, I generally try to eliminate as much risk as possible by dealing with known entities. Shelby fits that bill for me. I've also learned that his pricing on GTs is a pretty solid indicator FMV. It seems to me that he buys his cars with intelligence and sells them fairly.
So, there are always going to be a few GTs out there for sale with a wide variance of stories associated with them. I watched a couple of them sell at Barrett Jackson a few weeks ago, and they were both very nice cars that were right on the money, in my opinion. The market, however, is so tiny that I really wonder whether it should be called a market at all. Every car is different, and every buyer has their own agenda. My take, however, after doing a ton of research, is as follows: If a person is interested in a GT of extraordinary quality, then the number is going to be north of $300K. This is what I just paid. If a person is willing to take a few risks on an oddball car or one with a patchy history, the number will be significantly lower. It all depends on what a person is trying to accomplish and what motivates both the buyer and the seller.