GasBuddy.com says average retail gas prices in Illinois have risen five cents in the past week up to $2.34 a gallon. The national average is up 3 cents, to $2.25 a gallon. The average per gallon in Decatur is $2.19 and the lowest price per gallon in Decatur is a $1.99. The lowest price per gallon in Champaign is $1.97 and the lowest per gallon in Springfield is $2.03 a gallon.