United and American both have fares as low as $40 each-way between Chicago and a number of cities. Available destinations from Chicago include: San Diego, Los Angeles, New York LaGuardia, Dallas, Atlanta, Boston, Baltimore and Philadelphia.
There are limited dates available for each city pair and this deal won’t be available on all dates. Fares like this normally sell very quickly, so it is probably prudent to act fast if you see a date/route that works for you.
I used ITA Matrix to find the fares, but you should be able to book directly with the airline and with online travel agents such as Orbitz or Expedia as well.
For more information and analysis, see this Dan’s Deals post.