Any CPAs, Tax Pros or Oil & Gas guys have any resources for determining the histrorical value of royalty interests? I have a client that recently sold a large stake that was inherited decades ago. I am still hoping that there could be an estate return (that includes a valuation of assets) that will make this tax planning easy on me but experience tells me that will not be the case. Any ideas?

Was there a 706 filed? If not, you can get to a narrow range for basis based on a few data points.

What year was the death?
Lookup the Federal Estate Tax exemption for that year.
Consider if any of the exemption was used via lifetime gifts.
Reduce by values of other known assets (liquid assets, house/land, etc… that is easier to value.

If you get down to a small enough number, which is likely if deaths was 25+ years ago, the tax difference between using that difference as basis and using $0 as basis won't be too far apart.

There are ways to do historical valuations, but if dealing with a narrow band, just picking a mid-point will save costs.

Not confident in this, but I don't think there is a statute of limitations for IRS to come after the family if no 706 was filed. So, if you worry there was more value in the mineral rights, and want the basis to be higher to minimize costs, get a lawyer/CPA experienced in these types of matters.

Have explored doing an historical valuation before. Wasn't deemed necessary, but can be done.

Check the probate for an inventory.

They may not keep records this long but I would try to call the county tax office to see what the assessed value was for the year in question.