The energy content in fuels is often measured in BTU (British Thermal Units).
I'm assuming that you're talking about compressed natural gas, as in the form that is used in some cars like the Honda Civic GX.
One gallon of compressed natural gas contains 138,700 BTU.
One gallon of gasoline contains 125,000 BTU.
[http://www.bts.gov/publications/national_transportation_statistics/2002/html/table_04_06.html]
There are 1 therm in every 1 ccf of natural gas. Therefore, 100 ccf of natural gas is equal to 100 therms.
1 gallon of gas is equal to approximately 3.785411784 liters.
One hundred cubic feet (ccf) of natural gas is approximately equal to 1.037 therms. This conversion can vary slightly depending on the specific energy content of the gas, but 1 ccf generally translates to about 1 therm for practical purposes.
In 1995, the US national average price of a gallon of regular gasoline was $1.15. That is equal to about $1.65 per gallon in 2010 dollars.
In 2007, the US national average price of a gallon of regular gasoline was $2.80. That is equal to about $2.94 per gallon in 2010 dollars.
In January 2002, the US national average price of a gallon of regular gasoline was $1.139 - equal to about $1.38 per gallon in 2010 dollars.
In 1994, the US national average price of a gallon of regular gasoline was $1.11 - equivalent to about $1.63 per gallon in 2010.
In 2000, the US national average price of a gallon of regular gasoline was $1.51 - equal to about $1.91 per gallon in 2010 dollars.
In 1970, the US national average price of a gallon of regular gasoline was 36 cents. That is equal to about $2.02 per gallon in 2010 dollars.
In January 1996, the US national average price of a gallon of regular gasoline was $1.129. That is equal to about $1.57 per gallon in 2010 dollars.
In 1981, the US national average price of a gallon of regular unleaded gasoline was $1.38, which was a lot for gasoline in 1981. That is equal to about $3.31 per gallon in 2010 dollars.
In 1972, the US national average price of a gallon of regular gasoline was 36 cents. That is equal to about $1.88 per gallon in 2010 dollars.