Crazy waterfront property valuation

[William Rifenburgh]

Hello

I am considering buying a waterfront property and building a home on it. The problem is that the owners seem to want way too much for it. Theyr'e asking for about $180k for a vacant waterfront lot when the home next to it (also waterfront) has the same acreage, has a 1900 sqft house built in 1992 on it and has an estimated value of $210k on zillow. There are no other no other notable differences in the lots. How can the seller possibly justify such a price? Is this common for waterfront land pricing? What would be a fair asking price for the lot? 

Links to good info on vacant lot valuation appreciated :)