I don't know if I would buy in to the "appraiser's" notion. I tend to follow NADA guides primarily due to the fact they are what the dealer's that I know go by for their own needs. These little orange books come out every month and recap actual SALES figures for the previous 30 days. It seems to me that their site is updated monthly as well.
I took this paragraph below from their site for the sake of discussion...
"We cannot speak on behalf of other guide valuation companies; however, our editors review over one million sales transactions per month to help support our valuation numbers. These transactions come from many sources and depict actual sales both retail and wholesale. Because of how our values are created and the volume of data we review, NADAguides values are used across all segments including consumers, dealers, lenders, government tax authorities and insurance companies. All values are only a guide and prices will always be above or below the guide values for vehicles. The overall condition, mileage, history and local supply and demand contribute to the asking price of each vehicle."
I took a look just now at both NADA's and Hagerty's figures for a 1969 280SL. If you take NADA's "Average Retail" ($53,790) and compare it to Hagerty's average ($54,622) that's too close to be a coincidence IMHO; so I would tend to think that they are both based on sales, not estimates.
On the higher end cars, I would think that placing a value would be more difficult. My dealer buddy tells me that NADA really doesn't speak to #1 cars since for all intents and purposes, these cars aren't driven. It may be just that a small handful of cars are driving these figures but this said, there obviously are Pagodas selling for over $100k.
All in all, these guides are only valuation tools and meant to be guidelines.
One additional thought... it so happens that for our own amusement, my brother and I have been attending some of the big collector car auctions every year since about 2006. Its apparent that "muscle car" prices have certainly cooled but not so much European cars and exotics if you will. Seven years ago $750K was a strong price for a Gullwing but now the number is probably at least $1.25M. So with that type of increase, it only stands to reason that the next generation cars (ours) should be increasing as well. I'd be willing to bet that if the world economy grows, nice Pagoda's will be $250k five years from now. Lets face it, nice 190SL's are selling at that level now and they cant get out of their own way!