GasBuddy.com says the average price of gas in Illinois has fallen a little more than four cents in the last week as it hovers right around 2.21 a gallon which is exactly the same as the national average. The average price per gallon in Decatur is $2.08 and the lowest price per gallon in Decatur is $1.95. In Champaign, the lowest price per gallon is $1.85 and the lowest price per gallon in Springfield is $2.04.