The last I saw was about $2000 per acre, but don't quote me on that.
The offers may vary. They will vary because people get stars in their eyes. I had a fellow call because he was trying to buy mineral rights with a 26 acre place. The asking price premium to include them? $416,000....$16,000 an acre. No nearby production. OTOH, some companies are buying mineral interests at closer to market premiums - $2000-3000 in reasonably hot areas, but within recent production it has run $4000-7000/acre in the Fayetteville play.
Honestly, the lenders don't have a clue about minerals anyway.
That goes for most appraisers, too. It is not a position of strength to argue same however in light of the Competency provision of USPAP.
There is too much speculation about the value of minerals to safely (from a risk standpoint) extract values from the market place like you suggest. But the method you suggest (paired sales) is the correct and most defensible one to make.
The point is that the value of the mineral hinges as much upon your (the appraiser) credibility as it does upon the data. I certainly would try to take a mineral rights course as "proof" that you are trying to achieve competence.
Further, the bankers, the borrowers, the state boards need an education upon what the situation is. They need to understand that these values are much less stable than land values per se. This is largely due to specuation. It is like the condo market was. Anyone who recalled the collapse of the Mountain condo craze of the 1980's should have been most wary of the Florida condo craze of 2005-6.
I have seen bonus money paid, royalty rates paid, and sales of minerals sell for far more money [adjusted for inflation] than I could ever imagine sane oil men paying during the boom years of 1975-1982. There has never been a speculative craze like the Barnett and Fayetteville plays. And look out PA and WVA, the Marcellus shale is shaping up to do the same.
The risk for the appraiser isn't that you are wrong. It is that you cannot prove you are right. The risk is much less from the banker than from the borrower or a buyer/seller. Imagine using paired sales to determine that mineral rights under a 10 ac. tract in the Barnett is worth a premium of $30,000. The sale consummates and almost immediately the buyer flips the minerals as severed for $80,000? Then what? The seller thinks..I wuz robbed! Never mind that you have two side by side markets - one for land with minerals (your local buyer-farmer-resident) and one for minerals (professional mineral buyers usually). Again, you are fighting from a position of weakness. I certainly would ask a very large premium for incluiding the mineral rights if I were a res. appraiser.