The average retail cost of a gallon of gas in Maine is down by another nickel. Price-monitoring website MaineGasPrices.com reports Monday that gas has dropped to an average of $3.53 per gallon. That’s just three cents higher than the national average, almost 22 cents lower than a month ago, and nearly 41 cents lower than at the same time last year. The dropping prices are unusual, as prices usually rise in the spring. The Maine prices are based on a survey of more than 1,200 gas stations across the state.