BOSTON — The average cost of a gallon of gas in Massachusetts has fallen another 7 cents.AAA Northeast reported Tuesday that self-serve, regular has fallen in the past week to an average of $2.27 per gallon.That's a quarter lower than a month ago and $1.18 lower than a year ago.Massachusetts prices are also 12 cents lower than the national average.AAA found self-serve, regular selling for as low as $2.06 per gallon and as high as $2.89.