I found when reading about gasoline prices, that Illinois is sort of a "gasoline-market Island", California is also one as well as the Milwaukee area. Basically the state governments set regulations about what type of gasoline is sold in their area or state, and since there are few refineries that can make the stuff, as well as a limited pipeline system for delivery, that anything that affects the production or delivery will cause the prices to surge. So they get a specialized gasoline formulation and it has to be transported to them and distributed. So besides all the other factors affecting gasoline prices they also have more expensive special gasolines made for their areas too.
ref http://auto.howstuffworks.com/fuel-e...gas-price2.htm