I hate to sound like a jerk, but who or what is to determine fair market value? Depending on how bad you want something will determine how much you are willing to pay. I had zero interest in owning at Grand Solmar, or any timeshare in Mexico until I stayed there. Once we stayed at Grand Solmar, saw the accommodations, the grounds and researched the company I became very interested. After attending the presentation to find out more (developer costs, resale owner limits, and usage) I was on a mission to find one on the resale market.
I ended up getting a very good deal, I bought an EOY 1 BR during the Red Season. I was able to negotiate a reasonable deal because I believe I was reasonable. There is a mandatory transfer fee based on the size of the unit in question that must be paid to the developer or their accounting company. In our case, the transfer fee was $935.00 This has to be paid independent of the purchase price negotiated between buyer and seller.
Do the prices you have seen include the transfer fee? What is the size of the unit? What season is it for? Is it annual or EOY? How many years are left on the RTU? How bad do you want to own at Grand Solmar at Lands End? The answer to these questions along with other's desire to own there will usually determine if an asking price is reasonable.
I keep using the word "reasonable" because the word "fair" is in the eye of the beholder. For example, If the seller paid the Developer $23,000 for a unit that they only used for 3 years they may feel that an asking price of $18,000 would be "fair". However, on the resale market asking $18,000 would not be considered "fair" or FMV.
As stated before, I got a great deal in my opinion. This is because I would have been willing to pay 3 times as much as I did for the property.