For all the major retailers (Brooks, Saks, Nordstrom, Boss, Barney's, Bloomingdale's) I would suggest waiting until sales. You can get nice suits for 30-40% off at the typical Jan/Jun sales. As someone who lives and shops in Chicago frequently, I would find it hard to pay full price for most things (suits included). Sometimes paying full price is more of a necessity at a place like Paul Stuart or with timeless classics (your solid greys, navy blues, etc). With Off 5th, Nordstrom Rack, etc., it's a total crap shoot (often a bust, but sometimes you strike gold). So all of that depends on your time frame.
The Sales Associates at the Saks men's store on Michigan Avenue are probably some of the best dressed guys out there and that Saks carries everything from Boss to Brioni, so you have quite a wide range of prices to choose from, however minimum prices are probably $600.
But it also depends, maybe you don't like shopping at big department stores and prefer more intimate experiences that can be had at places like Syd Jerome or Morris & Sons.
As some of the previous posters have indicated, you're probably not going to get anything spectacular MTM / custom in Chicago in that price range.