This is because the two cities are so close to each other that at least in tech they're essentially the same job market.

I live in Milwaukee and disagree. There is no tech scene here. Programmers work for banks, insurance companies, Time Warner Cable, manufacturing, etc. if they work at all. In Chicago, there are actual software companies and consultancies. You would never see something like ThoughtWorks or Matasano (and their correspondingly competitive salaries) in Milwaukee.