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]
1 gallon of gas is equal to approximately 3.785411784 liters.
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 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 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 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.
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 1978, the US national average price of a gallon of regular unleaded gasoline was 67 cents. That is equal to about $2.24 per gallon in 2010 dollars. Also, you could still buy leaded gasoline in 1978, which was 63 cents per gallon.
In 1953, the national average price of a gallon of regular gasoline was 29 cents. That is equal to about $2.37 per gallon in 2010 dollars.