This article proves that it can and will cost more on ULCC's when compared to LCC's (Southwest Airlines specifically) By the time you add in all the fees, option charging, and nickel and diming the ULCC's prices will indeed be more than Southwest's all in ticket prices. Mainly due to Southwest IS the only airline that still fly's bags for free with NO additional upgrade charges or any fees to carry luggage period. Also, as indicated by this Frontier article, the ticket prices will be advertised as cheaper until you add in all fees, upgrades and charges to get you and your luggage from A to B. Highlighted para's bolded by me.
Southwest Airlines is still the KING of low fares and always will be, even up against the ULCC's
While opinions will always vary, and people tend to only be inclined to leave a review when they are irate, low cost airlines are generally not known for their comfort. Instead, people take these flights for their rock bottom prices, which can make up for all manner of inconveniences. Frontier...
The Denver-based Frontier Airlines will sell tickets that can sometimes cost as little as $19. But the reason it can get away with that price is that the airline will then proceed to charge you, often quite exorbitantly, for literally everything else.
A seat with extra legroom can run up to $25, and it can cost up to $99 to change or cancel a reservation, according to Sky Scanner. Even a simple can of soda or a bag of pretzels, which are usually included with ticket price, will cost a few extra dollars, as will confirming your reservation or selecting your seat.
But the main way that Frontier will get you is its baggage fees, which can often be even more expensive than your plane ticket. The Points Guy notes that “out-of-pocket costs for customers quickly go up when everything from bringing a full-size carry-on bag for $50 to $89 to traveling with a checked bag for $55 to $96 is factored in.”