Mineral rights can have a wide range of value. Ultimately, how do you determine what the value is?  Some issues to consider before selling your rights:

Price of gas present and future?

Price of other liquids extracted from the wet areas?

Who is the true buyer and who can they sell to?

What strata are covered?

How much are you selling?

How long will the well produce?

What are the tax and estate implications?

What happens if you get divorced?