To find land value, to to the mls, zillow or trulia. Find houses in that area that have sold withing the last 6 months to a year. divide the sell price by 4. In many parts of the country if you are dealing with local residential land you can find comps pretty easy. If you cant find other land that has sold, just look for houses. How much is an Acre of Land worth is an article that will give you more reference about land valuation.
The average price per acre is $10,000 or more if the oil gas and mineral rights convey with the property. If the rights do not convey The price is #3000 to $4000 per acre. Smaller parcels may demand higher prices.