OK, so what is the price?
gasoline in my area, $3.60 a gallon or so.
commodity NG was $5.00 or so
the last time I looked. (1MM btu)
would anyone else like to help?
If you want the real numbers for your area, you need to use the price of natural gas in your area. You won't get it at the commodity price for residential use.
You need to compare the price for equal number of BTUs. Gasoline is typically around 114,000 BTU's; it varys for summer/winter and other blends, reformulated is less, ethanol reduces it as well. 10% ethanol gasoline is 112,000 BTUs per gallon.
Natural Gas has 1,020 BTUs per standard cubic foot. Sometimes it is sold by the thousand or hundred cubic foot. Sometimes is is sold by the therm, which is 100,000 BTU, slightly less than 100 cubic feet, but ~2% difference.
So 1 gallon of 10% ethanol gasoline equals 1.12 therms of natural gas or 110 standard cubic feet.
The average US residential price for Natural Gas was $9.24 per thousand cubic feet this past January.
http://www.eia.gov/dnav/ng/ng_pri_sum_dcu_nus_m.htm
At that price, the gasoline equivalent will be $9.24/1000 x 110 = $1.02.
If you buy from a commercial filler, they will buy the gas at a lower cost, but charge you their expenses of compressing, equipment and profit. If you buy the compressor for your house, the electricity will be a small cost to compress it up. I've seen home compressors using 3 hp motors to give you some idea.