Believe it or not, these aren't the lowest fares we've ever seen on Allegiant (they're sometimes as low as $9, but only for a few travel dates here and there). However, we'll take 'em. They're also selling packages with buy 3, get the fourth night free. See the sale. And while you're at it, see the fees. Allegiant charges a fee to book online as well as to book over the phone. Avoid these fees by buying at the airport. Purchase by Jan 27 for travel completed by Sept 30. Offer not available Feb. 12–15, March 31, April 1–2, April 11–12, and April 18. 7–day advance purchase required.
You can only book Allegiant's fares on their web site, over the phone, or at the airport, not on third party sites.