Operating cost will vary by region, by gas utility, and by fuel type. Based on national averages, a natural gas fireplace consuming 27,000 BTUs/hr will cost less than 20 cents/hr to enjoy. LP units run slightly higher. Your monthly gas bill should include your exact cost per therm (100,000 BTUs). Based on this rate, and the BTU input listed on the rating plate on your appliance, you can calculate the cost for your area.