Peter — I’m guessing there was not a federal estate tax return but what about a state of Oklahoma estate return? Assuming none. Also assuming you are an Oklahoma resident. In the past during the days of Oklahoma estate taxes and valuations, I have seen valuations placed from 3 years to 6 years of actual royalty receipts accepted by both the State of Oklahoma and IRS as a basis depending on whether you wanted that basis to be low or high. I would use the above as a starting point for tax basis. I agree with Rick Roberts, your best bet will be to contact a local attorney such as Harrison to help establish that value. In addition to the taxation angle, the outright “sale” of a mineral interest in Oklahoma is usually considered to be a capital gain asset subject to very low rates (15%) on the federal side and excludable 100% from Oklahoma taxation. As in all things taxation, subject to change without notice and your mileage may vary – grin – Be sure to make sure your situation qualifies for the exclusion and capital gain treatment.