I haven't read everyone else's replies yet but one should keep in mind that the appraisal will only show fair market value in it's current undeveloped state, not the expectation of what the land will be worth if a marina was put in or a golf course or a hotel was put in. In other words, no improvements can be included. The closest comparables are 30-50 miles away, maybe more, in areas that are much more developed.
I think the appraisers will have to check out other large vacant tracts for sale. I seem to recall that more inland tracts (wine country were asking for an ungodly amount and had no takers ($10MM for 370 acres for example in one case and $10K per acre in another case). I'd expect something in the $20 million range for 5100 acres on the ocean. But that's just a guess based upon two years I did appraising (a long time ago).
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