Valuing Mar-A-Lago is a difficult proposition. Absolutely unrestricted, it probably is worth $300 - $400 million based on comps in the area. That is particularly true if it could be subdivided.
But it is not unrestricted, and as is I'm not sure it is worth anything. As long as it is limited to use as a social club, which is only bringing in about $20 million a year, it is a massive money loser. Comps in the area are irrelevant due to its restricted use, as well as the factors cat mentioned.
It is worth noting that in 2020 Trump had henchmen argue that the assessment of $26+million was too high in order to reduce his $511K property tax bill.