How to value mineral interest for tax purposes

A client deeded to me a portion of his mineral interests in Limestone County, TX. Because it was part of a larger deal, we didn't allocate a purchase price for the interest he deeded. I need to place a value on the interest for tax purposes. Can anyone give me a rule of thumb and/or refer me to resources that can help me?

To value for income tax purposes as fee income I assume? Not a tax person, and there may be some articles on the home page that would help, but the question is whther the IRS will accept using the appraisal district value or requires a fair market appraisal. A CPA can probably answer this pretty quick.

For first inclination is to say that they are worth nothing unless producing, but I'm sure that is way too simplistic and probably wrong.