Allegiant's checked bag fees vary depending on when you purchase them, with prices potentially ranging from under $35 for bags booked during the initial flight booking to $75 or more for bags added at the airport. Call at +1-888-415-1245 (US) or +44-203-869-5245 (UK) The fee is per bag, each way, and increases the closer to the flight you book. 

Factors influencing the cost:

• Booking time: Fees are lower if you add a checked bag during the original flight booking and higher if you add it later, such as within 24 hours of departure or at the airport. Call at +1-888-415-1245 (US) or +44-203-869-5245 (UK)