Gary,
I think you may have gotten a "rule of thumb" appraisal not one based in science and economics particular to your mother's mineral rights. Without proven reserves (at term of financial art) your mother's minerals only have speculative value which must be strongly discounted in a present value determination of fair market value.
Feel free to PM me if you have questions.
PS I manage minerals in McKenzie County continuously and can tell you that Mr Kennedy has his finger on the pulse. A scientific based risk discount rate is what need be applied.