Appraisal of Mineral Rights

My accountant is urging me to get an appraisal of the mineral rights. I recently sold our farm in Gonzales County but kept 70% of the mineral rights. He says it would save money in the future. I think this will cost $2000 - $3000. I'm unwilling to spend the money since we are not currently seeing much activity in our area and may never get anything out of it. I have an appraisal of he land when it was sold a few months ago, but not the minerals. Do any of you have any information on this subject or do you iknow who I should contact to get such information? Thanks