GasBuddy.com says average gas prices in Illinois rose 7.1 cents in the past week – up to $2.35 a gallon. The national average is $2.21. The average in the Decatur area is $2.15 a gallon and lowest gas price in Decatur is $2.06 a gallon according to GasBuddy. The lowest price per gallon in Champaign is $2.03 and in Springfield $2.15 a gallon is the cheapest.