There are lots of companies out there offering to buy mineral rights. In my experience, their first offers are usually low to what they have been authorized to offer. They would rather buy for a lower amount than a higher amount. The area of Dunn would be important to know as the geology is key to the offer amounts. Can you give the section, township and range?
Also, they should talk to their accountant as they may have capital gains on any sale. If they pass the minerals on to heirs, the fair market value has a reset and the next generation may not have as high a capital gain if they wish to sell. And in the meantime, your folks get the royalties, if any.
Your folks need to find out if there is pending activity in their area. Most offering companies want to make money on any deal that they do, so they usually don’t make offers in dead areas. They want future royalties.