You think there's stuff equivalent to Mar-a-Lago in the area? Look at a map. It's the only thing like it in the area. Sure, there are mansions nearby, but those aren't the same as Mar-a-Lago that is as much an exclusive resort club, as a residence.
Do you estimate the value of it as private property and a single residence, or as commercial property? How much is the undeveloped (used for something like tennis courts for example) land within it worth if some of that were sold off separately as a lot?
The tax assessment is a nonsense figure used for tax purposes, not an accurate valuation of the property as real estate for sale.