The average price of a gallon of regular gas in Sonoma County is still trending downward. It’s $5.17 today, down one cent from Sunday. It was ten cents higher one week ago and 31 cents higher one month ago. The statewide average is back below the five-dollar mark, coming in at $4.98 per gallon today. The cheapest gas in Sonoma County is at the Costco in Rohnert Park where a gallon of unleaded is $4.75.