Value of inherited royalty/mineral rights Sec 21, Block29, PSL, Loving County

Inherited royalty/mineral rights in Section21, Block 29, Public School Land Survey, Loving County TX from grandfather with a respective royalty rate of 0.00234375. I’m one of eight siblings from my mother and uncle with royalty rate of 0.00029297. The four siblings of our deceased mother sold the Royalty/Mineral rights for $60,000 or $15,000 each. Our mother passed in December 2007. What value do we place for capital gain purpose in December 2007?