Frontier and Spirit have round-trip flights from Dallas (DFW) to Chicago (MDW/ORD) for $52-$54, NONSTOP. $26-$27 each-way. Flights August to September. Departures on Tuesday, Wednesday, and Saturday. Extended weekends available. Origin/Destination reversible.
These fares do not include a free carry-on bag. Only a small personal item, like a Jansport SuperBreak backpack is free. You can avoid paying for seat selection if you accept the randomly assigned seat prior to boarding.