The average price of a gallon of self-service regular gasoline in Riverside County rose a half-cent Monday to $4.266, a day after dropping three-tenths of a cent. The average price has dropped 14 of the past 15 days, decreasing 7.4 cents, including six-tenths of a cent Saturday, according to figures from the AAA and Oil Price Information Service. It dropped 11 consecutive days, rose two-tenths of a cent Thursday and resumed dropping Friday.