114,000 BTU in a gallon of gasoline. 75,000 BTU in a gallon of LNG. You usually measure fuel heat content in $ per million BTU to simplify comparisons on an equal basis. So that's 35.09 $/MBTU for Gasoline and 26.67 $/MBTU for LNG. Hence, LNG is 24% cheaper than Gasoline. Plus you'll probably save a lot of money on reduced maintenance costs due to LNG being very clean, so $/ton-mile will be more than 24% cheaper.
http://en.wikipedia.org/wiki/Gasoline_gallon_equivalent