A decline curve can show you how long a well is projected to last. If you project the price over that period then you can estimate the income and even the “Present Worth” (If you are an engineer) or the “Present Value” (if you are an appraiser - same thing pretty much). I would think the “several hundred” dollars in cost would be the low end of the scale. A typical appraisal fee of the mineral right (assuming a certified appraiser which is a bank requirement usually) might be from $2,000 up.
PS - mortgaging your mineral rights to be supported by the income can be dangerous as a well could be shut in to perform maintenance or simply have a mechanical failure. Be sure you can service the debt without the income from the well.