I fly 7-8 times a year from Chicago. Since Mexicana went under, the prices have stayed over $400. I did find a replacement flight on Spirit Air for my suspended Mexicana flight that totaled $290...I carried a personal item back and returned with carryon and two checked.
My advice is to go to
http://www.kayak.com, start an account, and get daily fare alerts. Your dates are still before the Christmas rush. Decide what you need for luggage, arrival/departure times, and other considerations. You can go to Spirit Air website and check, they will give you a calendar with prices. Usually it is less money than what appears on the kayak search
I thought I heard that Taca may add flights there. I track flights daily on Kayak with many dates selected. An overview:
Aeromexicanas and Spirit Air prices on kayak are misstated.
American Airlines is around $425 with 2 free checked bags and breakfast, lunch or dinner on the SJO - MIA/DFW segment. Price jumps $150 -$200 within two weeks of departure date. AND
you can hold a reservation for 24 hours. That's what I do when I'm closer to booking.
Delta has 2 free checked bags
Continental has 1 free, charge for 2nd
US Air has charges for both bags, but allowed two free return. Go figure.
Taca dinner, lunch booze 2 free checked but a stop or two in Guatemala or El Salvador.
I booked my 3 month flight at $255.00 Delta in July. Then the prices stayed at $300 until Mexicana suspended all flights. Then they went to $400, staying at $425 range since.
Delta flys to both Midway & Ohare I booked an early morning departure at Ohare and a return to Midway.
I booked yesterday with AA because I booked a Friday departure, it offered more than one arrival time in Chicago (Delta only arrives at 10:30 PM) free checked, food, and price. Prices jumped $15.00 for most carriers on 1 day.