A quick calculation using your monthly gas bill and BTU from the lantern will yield the monthly cost for running a lantern.
The cost for running a lantern can be calculated using the following equation:
((BTU per hour use of lantern x 720)/(102000)) x CCF
Where CCF or CHF is the cubic hundred feet charge found on your monthly gas bill, and BTU is the quoted gas usage from the lantern.
Example: Copper Lighting Co. lantern uses 1460 BTU per hour and my gas bill has a .64 CCF rate: ((1460 x 720)/(102000)) x .64 = $6.60 per month per lantern when run 24 hours a day, 7 days a week
Our typical BTU ratings/hour by manufacturer can be found here.