The cost to fill is continuing a downward trend in Montana.GasBuddy.com reports average gasoline prices in Montana have fallen 6.3 per gallon in the last week and sat at an average of $2.98 per gallon as of Monday, Nov. 18.Gas prices are 20.5 per gallon lower than at this time a month ago and 31.9 per gallon lower than a year ago. The national average price of gasoline fell 0.7 per gallon over the last week to $3.02 per gallon, which is 15.2 lower than a month ago.GassBuddy.com reports the cheapest gas in Montana was at $2.81 per gallon on Sunday while the most expensive was $3.39 per gallon.