Spirit has round-trip flights from Chicago (ORD) to Cancun (CUN) for $190-$199, NONSTOP. Flights in September and November. Origin/Destination reversible.
Note: Spirit fares do not include a free carry-on bag. Only a small personal item, like a Jansport City Scout backpack is free. You can avoid paying for seat selection if you accept the randomly assigned seat prior to boarding.
Note: Spirit seats only have 28-inches of pitch; 2-to-3 inches less space than most other airlines.