GasBuddy.com says average retail gas prices in Illinois have dropped 6.3 cents in the last week – down to $2.27 a gallon. The national average dropped 2.9 cents in the past week, to $2.19 a gallon. The average gas price in Decatur is $2.11 per gallon and the lowest price per gallon in Decatur is $2.06 a gallon. In Springfield, the lowest price gallon is $1.99 and lowest in Champaign is $2.06 a gallon.