I have done appraisals for a number of estates in Colorado, Texas and Louisiana. I have some simple rules of thumb that have served me well. Generally, I give very little value to unleased minerals, unless the deceased recently rejected a bona fide offer -- in which case, that offer can be a floor valuation. If leased, but not producing, then I value the minerals at 3-5 times the bonus paid, depending on a number of factors. If the minerals are producing, I value them in the range of 36 to 60 times the average monthly royalty (in the case of the shales, you have to consider how long the property has been producing, because in the first year, a 60 times multiplier will over value the minerals.
Personally, I retain all the crazy, unsolicited offers to buy my minerals or producing royalties. These offers could form a basis for valuation as they come from independent parties with industry expertise.
And finally, Kristi. You fear your family is going to be stuck with inheritance taxes on the mineral rights. Keep in mind, at least in the case of Federal Estate taxes, it is the Estate that pays (of course, ultimately that is money the heirs don't receive). For 2013, the Estate Tax exemption is $5,250,000, so this is only an issue if the total Estate exceeds that amount. If it does, then the objective is to keep the mineral appraisal as low as possible. And to emphasize my point again. If the minerals are not leased, they likely have very little value. This methodology is consistent with states that have property taxes on minerals (like Kansas and Texas). They only have taxable value if they are producing. West Virginia, on the other hand taxes minerals regardless of production status.
I hope this helps.