The cost of a gallon of gas in Maine has plummeted nearly nine cents in the past week. Price-monitoring website MaineGasPrices.com reports Monday that the average retail price of a gallon a gas has dropped to $3.79. That’s still 12 cents above the national average. Current prices in Maine are now nearly 16 cents per gallon lower than a month ago, yet remain more than 24 cents per gallon higher than at the same time last year. A company analyst says there hasn’t been such a significant price drop in one week since May 2011.