Allegiant charges $15--50 depending on when you pay. https://www.allegiantair.com/popup/optional-services-fees#ba...
Frontier charges $30--60 depending on when you pay. https://www.flyfrontier.com/travel-information/baggage/#info
Spirit apparently charges $35--100 (according to many news and other sites online), though maybe the last price is "on the airplane" only?
Even Ryanair doesn't charge for carry-ons.
Even Ryanair doesn't charge for carry-ons.