Apartments in Chicago area
I'll throw my .02 in here,
I've been commuting from the Oak Park area to DPA for almost 2 years, with a brief stint in MKE last summer. Just moved out to the Clarendon Hills area - you might want to try looking around there (Westmont/Downers Grove/Darien), also, friends of mine rent a finished basement in LaGrange for about $600/month.
Chicago Ridge, Worth, Alsip and Blue Island (to second what people have already said), but you might also luck out in Oak Lawn - maybe try close to the expressway (294).
$500 might be tough, but when I was looking in the area I am now, I found studio/one bedrooms in Glen Ellyn for not much more than that. That's a real easy commute to DPA - right down North Ave (if only it wasn't under construction).
I have no idea if there are apartments in the far Northwest - Hampshire/Marengo area - but if you're considering driving all the way from DeKalb, those are just as far and might be cheaper - but I'm not sure.
Hope this helps, happy hunting,
-brew3