Frontier (flyfrontier.com) has a nonstop for the return, on Jan 15, with 4 seats left @$102 per seat, for members of their Discount Den program. They will charge you for carry-ons, checked bags, beverages, etc., but it would still be less than Southwest. They also do not have the liberal change/cancellation policy of Southwest, but, what other carrier does?
Discount Den info:
BENEFITS OF DISCOUNT DEN
Access to our lowest fares
Receive special promotions
Book Den Fares for up to 6 people
Get the savings all year long
ENJOY ALL THE BONUSES!
Become a Den member for only $49.99/ year.
Note: I do not believe that Frontier participates in Disney's resort airline check-in, where you can drop your bags for the return flight right at your Disney resort. You should, however, still be able to ride the Magical Express to the airport.