Gas prices are down in Michigan to start the new week. The average price for a gallon of regular unleaded gasoline in the state is $3.81. That’s down about four cents from a week ago. According to AAA of Michigan, the most expensive areas to buy gas in the state are Metro Detroit area and the cheapest fuel can be found in Grand Rapids. Locally, Port Huron area pumps are slightly above the statewide average, at $3.84 a gallon.