Check with the county assessor for the taxable value. Also compare land value of other vacant lots or homes with double lots near your home. The later is the best (subtract your lot value from the double lot value) But really it's about supply and demand and the demand for this property is 2. He's lucky you want to buy it otherwise it's just a tax liability every year. With this type of situation the price will probably boil down to who is more emotionally attached to having the property. break down the land value of the adjoining property by square foot and multiply that by the square footage of the lot. I would make this number the ablosute high number I would pay. My goal maybe to pay half that since it's not buildable and my starting point would be below that.
Nov 2, 2008