The problem with generalizations like this is that they are just generalizations, and so not very useful when looking at a specific car.Several prominent builders have quoted me 95K for new basic build. Add another 10K for more glamorous interior. Frankly I could not do it for that. But someone can. In the end they are only worth what someone is willing to pay. I know y'all are proud of the cars and the effort and rightful so but 85K is about right.:shrug:
Just as with normal cars, the cost for the "same" car can vary widely in terms of initial cost. Someone mentioned Porsche above, and that's a good case: you can buy a Cayenne for around $58K new, or easily blow it up to over $100K with options. Or start with the Cayenne Turbo S and with a few too many clicks on the options list it can get to $200K. An intelligent buyer looking at two Cayennes, one a stripper, and one loaded up with options would surely value the stripper car much less, or the loaded Turbo S much more.
The same thing applies to SLCs. They are all 1-of-1 cars that happen to begin with the same base. Then people put in wildly different drivetrains, very different interior treatments, no paint or $20K paint jobs, brake and wheel packages, no or major body modifications, wildly varying build quality, etc.
That's why it makes as much sense to say "any 2015 Cayenne is worth about $50K used" as it does to say "$85K is about right for an SLC". Neither statement has very much information value, because the cars are so different that each car must be valued on its own.
As Bill mentioned above, there are reasons why any expensive used car will be harder to sell. In the case of an SLC, you need to find a buyer with $80-140K in cash or readily convertable assets, which already makes the buyer pool very small. Then you need to find one who feels comfy with the idea of maintaining a one-off car, which makes the pool much smaller still. Then you have to find that buyer who loves your car.
It's a hard thing, and like all sales, if you wait to find the perfect buyer, you'll get a fair price. In a hurry?-- you'll end up taking much less than you would have gotten had you waited.
I don't know what a fair price is for any one car, but the market for that car is what a willing buyer and seller agree upon. For another car, that's an interesting number, but because of the extreme variability of these cars, it's just one data point, and not necessarily indicative of the market value of the next car.