First, is the portion of your lot that you are going to sell a full buildable size lot with no variance required. If it is, then go to a local realtor, county or local assessor where sales are recorded and look up recent lot sales in your neighborhood. This comparison method will leave you with no doubt. If the lot is not sufficient to be built on, just a portion that a neighbor wants, it will be worth far less then a buildable lot. Use the same method as I described above but compare them to only non-buildable partial lots. They sell for far less then a buildable lot. Depending on your area and market demand, several mediums of measurement could be utilized. Price per acre, price per front foot, and price per square foot. Good luck, it is not hard.