The tax basis is going to depend on how you acquired the interest. Did you buy a OGL and sell it with a retained ORRI or did you inherit the interest. Either way establishes a cost basis. It seems you are trying to establish a market value rather than a cost basis. If you retained the ORRI and did not recover the full cost of the OGL, then the remaining unrecovered cost is your cost basis. If you inherited, then it would be the estate valuation for that interest. Market value is what a willing buyer and seller can agree to for a sale/purchase. Amounts beyound that are considered speculative valuations. I hope this helps.