My friend flies them all the time from Bellingham, WA to San Diego to visit me. Once a month they have fares as low as $19 each way! Once they had a sale and it was $9 each way! It is NO frills and if I recall correctly when I looked into flying them, they make up for it in charging lots of fees. From their site:
"Base prices do not include PFC, segment tax or September 11th security fee of up to $10.70 per segment. A segment is one take-off and one landing. A convenience fee of $14.99 per passenger will apply when booked on allegiantair.com. A convenience fee of $14.99 per passenger, plus $14.99 per segment, will apply when purchased through Allegiant Air call center. Purchases made at any Allegiant Air Airport Ticket Office will not incur a convenience or call center fee. When purchased at time of booking, a checked bag fee of up to $34.99 per bag, per segment will apply for the first two(2) checked bags."
That said, it's still much cheaper than most airlines.