Well, cheaper than it was at its all-time high earlier this month. AAA says the average cost of a gallon of gas in Washington dropped by 12 cents since July 6th. While that sounds great on the surface, don't get too excited...even the cheaper gallon of gas in Washington costs 27 cents more than the national average. The most expensive gallon of gas in Washington can be found in Bellingham ($4.27 per gallon), while the cheapest fill-up is in Vancouver ($4.13 a gallon).