Until the advent of Unconventional (Horizontal) Drilling in the areas of the "Lower 48" where Shale Deposits are located, you could generally depend upon one of the simpler calculations for the value of Undeveloped Mineral Interests or Developed (Producing) Royalty Interests:
You could see "3 times" the most recent Lease Bonus / NMA Rate for undeveloped interests as a beginning point for negotiating a selling price, or take the average of the most recent 3 months of Royalty Income from a property and multiply it by between 36 and 60 months (3 to 5 years) for developed (producing) properties.
But the World is not that simple anymore, especially in the Permian Basin. In some areas, a single formation can be as much as 900 feet thick.
If you have 442 NMA anywhere in Reeves County, presently producing or not, you need to consider engaging the services of an extremely experienced Oil and Gas Attorney to assist you in wading through the myriad offers you are receiving or will be receiving. And, perhaps just as importantly so, be willing to spend the money to have an experienced Petroleum Engineer prepare an evaluation of your properties.
I have not yet looked at your Section 8, C-18 yet and do not at the moment have any information about "Going Rates" in the area (other than Linton's offer of $21K per acre for her property, which is about 5 miles South of you and pretty impressive), but I have heard of acreage in Northeast Howard County, which is waay East, Northeast of you both, something like 5 Counties further out, going for $62.5K per NMA.
I don't know how to advise either of you further at present.
I've been thinking of Mapping Out what Lease Bonus' and Offers for Mineral and Royalty Interests have been over the course of the past few years in West Texas: Where, When and how much for what...
Appreciate your faith in me -
Charles Emery Tooke III
Certified Professional Landman
Fort Worth, Texas