I just bought my first ever connecting flight at the airport for Spirit. One way DFW to CLE, connect in MCO. Was $103 online, so I figured it would be $83 at the airport. Turns out that it came to $63. In other words, I saved the PFC fee on both segments of the the flight.
Dburns.... unless it has changed recently, it has always been cheaper to pay for bags online, not at the airport, even if that is the initial ticket purchase point.