I MIGHT be interested in selling my mineral interests/producing wells. I get several offers each month and have an idea of what they are worth. Is there a industry/trade marketplace to research? How else would I go about putting them on the market?
Take the average monthly payment of the last year (further back, beyond July 2008 if necessary,... you want to get that average higher), then multiply by 68, 84 and 100 months. That's a pretty good rough estimate and a spectrum of current value, BUT it doesn't take into account any reserve estimates or whether your royalties are attributable to gas or oil.
I recommend NSAI (Netherland-Sewell and Assoc.). They are a good third party to help you with this.