I am just wondering if anyone has tried to buy Allegiant tickets at the airport in an attempt to save money like on Spirit and Frontier?

With Spirit, you save $17.99 each way off of the non-discounted "regular" fare that anyone can buy online.

With Frontier, the savings vary based on flight. It is at least a $4 savings each way minimum, and $18 savings each way maximum. The only way to tell how much you'd save on a particular itinerary is to highlight the days you want to fly, then click on the right hand side of the "Airfare Total" screen where it says "Taxes, Fees, Government, etc". The resultant popup window will show the fare breakdown. The Carrier Interface Charge total is what you want to look for.

If this is for a roundtrip, and the total is $36, then you know you are saving $18 each way by going to the airport. The heavily discounted super short flights --CLE/PHL was the example I saw most often being based near Cleveland-- are usually $8 total roundtrip, for a rather meager savings of $4 each way.

For Allegiant, I did dummy bookings for both super short (LAS-REN) and long haul (LAS-CIN) flights, and the Carrier User Charge was $26 roundtrip both times. Keeping in mind that the online price is for debit card users, I just mentally figured an extra $5 or so for credit card purchases.

If one can indeed save money on Allegiant by going to the airport, the big problem for most people would be trying to get the airport when you can buy tickets, since Allegiant has such limited flights at most airports they serve.