Selling Mineral Rights: Setting a price

I’m about to sell my inherited mineral rights. I receive offers for them regularly, but the grapevine tells me those are for pennies on the dollar. How might one gather legitimate bids? How best can I establish fair market value for listing on an open market? Where IS that open market? While we’re talking value, how do I establish past value (for capitol gains taxes)? I’m guessing I should at minimum ask the companies currently working the wells. These are generic questions, though the minerals in question are in Canadian County, OK. Thanks for any suggestions (including almost surely those that question ever selling).