The least expensive gas in Grass Valley is six cents a gallon more than it was a week ago. Analysts say the rising prices are the result of the annual refinery maintenance season when supplies of winter blend gasoline drop as refineries start production of the more expensive summer blend. The six-cent spike is the biggest one-week jump locally so far this year. Despite the increase, gas prices in western Nevada County are among the cheapest in northern California. Triple-A does not survey Grass Valley, but prices here are about the same as they are in Marysville, which the latest triple-A survey says is the least expensive in northern California.