I've been purchasing my ticket at the airport counter for years. 2 Decembers ago I flew to Miami for $13. I flew from ORD.
Also I believe the $17 you save by going to the airport is per segment. So if you want to go to ORD -FLL with a layover in Atlanta, the first flight to Atlanta would be discounted $17 and then the flight from Atlanta to FLL would be discounted another $17.
I have had tons of flights that cost -9 cents before tax.