Good luck reconciling your royalty to market prices.
If you subscribe to Wall Street Journal, WSJ has a user friendly advanced charting for NYMEX gas price, although your local gas prices may vary greatly.
Ancova Energy (Ancova.com) has a free emailed-newsletter providing NatGas prices for Midcontinent, Permian, Eagle Ford, Bakken, Haynesville & Appalachia … along with commentary. This newsletter also prices & comments on oil & NGL prices & rig count by basin.