Expand your search on one of the nation wide sites like Car Gurus. Anyone here would be doing the same thing to find a price for you. As we've not see the example you are looking at we might miss an option and give you numbers that are low. There is always KBB and NADA for private party as well as dealer values.
I would go to your local dealer and get them to look up the VIN number of the one you are looking at. They can give you a printout that shows the original MSRP of the one you want, including its options etc, also the dealer invoice. I did this a couple of times while shopping around for used Range Rovers. If the sales people won't do it go to the warranty manager and ask about getting an extended warranty for it.