The cost of a gallon of gas in Maine is down by almost a nickel. Price-monitoring website MaineGasPrices.com reports Monday that the average retail cost of a gallon of gas has dropped in the past week to $3.59. Even though that’s almost 40 cents lower than at the same time last year, the price in Maine remains seven cents per gallon above the national average. The current in-state price is now almost 18 cents per gallon below where it was just a month ago. The average is based on a survey of more than 1,200 Maine gas stations.