Did a judge really say that Mar-A-Lago is only worth $18 million dollars?
An entity that the Trump's claim produces over $20 million dollars in revenue a year? As I've seen some say, it would mean the estate hasn't even kept up with inflation in terms of value. Let alone considering the surrounding real estate market in that part of the country, the improvements to the property over the years, and the revenue generation component.
Even taking all of this at face value, it's pretty easy to make the case that a lot of that income (most? all?) is related to Trump's presence there rather than the property itself. There are plenty of instances of folks paying top dollar for the chance to run into Trump either for political reasons or fandom. Market value would reflect that any purchaser would not be able to squeeze nearly as much money on a regular basis as Trump has.