GasBuddy.com says average retail gas prices in Illinois have fallen 8.1 cents in the past week down to $2.09, as the average price per gallon in Decatur is $1.86 with every gas station in Decatur as of today having gas under $2.00 per gallon. The lowest price per gallon in Springfield is $1.82 and $1.86 in Champaign.