BOSTON — The price of gasoline in Massachusetts has dipped slightly and remains well below the national average.AAA Northeast reports Monday that the cost of a gallon of self-serve, regular in the Bay State is down a penny from last week to an average of $2.17.That's 12 cents lower than the national average but 49 cents higher than the in-state price a year ago.AAA says the stable prices are due to lower demand at this time of year and increased U.S.