American Airlines has round-trip flights from Dallas (DFW) to Chicago (ORD) starting at $97, NONSTOP. $49 each-way. Flights in January to June. Friday departures are available for about +$6 more. Origin/Destination reversible.
Note: Cheapest flights are on Tuesdays, Wednesdays and Saturdays.
Note: Carry-on bag is allowed for free, but checked bag will cost about +$30 each way. Seat is assigned randomly, for free at the gate.