You can get a break down of the fare online. Allegiant's fee varies and on their cheapest fare it's a little bit less. Since Allegiant doesn't have coupon codes for online booking (as far as I can tell) a fare at the airport will always be a bit cheaper than the one online (not the case with other airlines that play the same game like Frontier or Spirit, since they have online discounts that are sometimes larger than the online fee).

I'm no expert however, but that's how I understand it.