Melinda,
It is difficult to place a well-defined value on minerals. The value is often dictated by the market, and what someone is willing to pay for them. However, unsolicited purchase offers often are significantly below what would be considered competitive. If you post the description of your property's location, I can pull it up on a map and let you know what is going on in that area.
You may also want to try reaching out to one of the forum sponsors to see if they have any historic sales data for properties in your area.
http://marketplace.mineralrightsforum.com/ad-category/minerals-management-consultants/