Well, not sure where they got those numbers.
20 October 2014, Spirit $163.29, Delta $232.10, unless you fly on the 0752 flight with a stop in CVG, then it's $1021.60!!
Even for the basic fares, Spirit is nearly $70 cheaper.

*prices run off of each airline's website at 1200 on 16 October 2014. Doesn't include baggage fees on Spirit.