Is $3750/ac a decent price to sell your mineral rights? Hmm. Do you need the money soon? Would you be angry with yourself if the buyer doubled their money in five years?
If you're inclined to do things yourself, figure out the ancestors of the grandfather, then look them up on landex. The more research you do on your own, the more info a professional can start out with. Find someone (I can give you a reference) to do the rest of the deed research. Then you will know if you actually own something.
Right now yes $3,750 is a very good price. And in five years prices may rise a bit, but NOT anywhere near what it was 2-5 years ago.