The best way to get a value on any type of real estate would be by getting a sold comparable through the MLS. If you cannot find a sold comparable the next best option would be a pending or an active comparable. Also, if you cannot find anything comparable at 7 acres, you can create a ratio of price/acre and use that.
For example: 2 acres of agricultural land sold for $100,000, therefore each acre in that area looks to go for around $50,000. A good estimate would be that your land would be valued at $350,000. There is much more research you can do though, through the MLS.
Sep 21, 2016