Hi, Kevin -
Several people here on The Forum have asked about how to value mineral interests. And I know that there are professional firms out there that prepare such evaluations.
Perhaps someone else can offer you more detailed information, but from my experience, if the mineral interests are not producing then for the sake of an estate or estate tax calculation, there is no value.
If they are producing, then I suppose you could take the average of the most recent three months worth of royalty checks and multiply that by anywhere from 36 months to 60 months. At least, that's the formula I would advise someone to use to arrive at an asking price when they are trying to sell their mineral interests.
Hope this helps -
Charles Emery Tooke III
Certified Professional Landman
Fort Worth, Texas