We did a Chicago/Madison trip this past week - coming and going through Dbq and starting in WDM.
Eastern Iowa is typically .15-.20 higher than central Iowa. I've always noticed that Ames seems about .08-.10 higher than DSM, but the oddity is that we travel hwy 330 through Albion, then over through Gladbrook and Traer into Waterloo then 20 over over to Dbq. Usually gas is about the same as DSM all the way to Albion/Gladbrook, even though it's similar distance from those supply lines. Then it's usually about .10 higher around Waterloo and +.20 in Dbq.
I think I filled up in DSM early last week about 2.25. It was in the 2.40 range in Dubuque, then once across the river it jumped to about 2.60. Of course nearer to Chicago it was more like 2.75-2.85. Coming back through Madison was closer to the eastern Iowa prices of 2.30 or so - by that time the prices here were more like the 2.15.
I find it kind of funny that most other cities seem to go higher in prices the closer you get to them, but central Iowa and Des Moines almost always as the cheapest gas in the state.