@blablarg18 You might have been right about the value of Mar-a-Lago if the purchaser of it could do whatever he or she wanted with the property; however, Trump has burdened the property with an extensive amount of land restrictions, conservation deeds, and preservation easements.
In 1993 - "Declaration of Use Agreement" with Palm Beach to obtain permission to use the property as a "social club," wherein he burdened the property with having to obtain governmental permission in order to use the property for any other purpose than the existing social club. So, if you wanted to use the property as a private residence, you can't without the explicit permission of Town Council of Palm Beach, the Landmarks Preservation Commission, the State of Florida, the United States Government, and any relevant agencies.
In 1995 - Trump signed a "Deed of Conservation and Preservation Easement" within which he gave up all rights in the property to use it as anything other than the existing social club.
In 2002 - Trump signed a "Deed of Development Rights," wherein he granted an easement over the property to the National Trust for Historic Preservation and agreed to forever extinguish rights to develop and use the property for any other purpose other than the existing social club. This Deed specifically prohibits changes to the property, including dividing it up, using any of the property for single family use, construction of new buildings, internal renovations inside the building, or obstructing any existing natural vistas. Trump likely signed this Deed because it allowed him a massive local tax cut.
If you read the actual opinion, you will see that the judge disputed Trump's SFC valuation and Trump's expert at trial because neither of them took into account the extensive restrictions on the use of the property in the valuation of it.
TLDR: Trump's values didn't account for land restrictions which are so restrictive that any purchaser could only be buying the property to continue running the existing social club. Basically, the FMV as land is irrelevant because if you bought the land, all you could do is keep running Trump's club.