The average price for a gallon of gasoline in the United States fell 6.4 cents in the last two weeks. According to the latest Lundberg survey, the average price was about 31 cents lower than a year ago, based on the Sept. 20 survey. A quick survey for the lowest price in Missoula came up with $3.56. Charleston, South Carolina, had the cheapest gasoline in the survey at $3.14 per gallon, while drivers in San Francisco, paid the most at $4.01 per gallon. Some industry experts believe the price could keep falling, even below $3 per gallon for the first time in three years.