How to find our historical value of mineral rights

Does anyone know a source for historical information regarding prices that mineral rights sold for back in 2002? We inherited some back in 2002 and sold part of it in 2017 and need to figure out the cost basis. There were no valuations as part of the settlement/probate of the estate.

Yeah, I guess I'll have the same issue if I sell... have to know the value when I inherited it back in 2011. I'v eheard that there are oil/gas land valuation folks, but have yet to track one down...

I've been speaking with a Keith Selinger out of Amarillo that has done it for many years. He was going to charge me "around $500" to research mine as of Dec 2013. He works for GSM Petrolium Services. If you want his contact info, let me know.