Budget airlines look cheaper because the first number they show you is smaller. Whether they're actually cheaper is a different question, and the honest answer is: sometimes, and it depends entirely on the trip you're really taking.
The base fare on an ultra-low-cost carrier is often genuinely low. That part is real. What changes the math is everything the base fare leaves out, and how much the trip costs you on the day something goes wrong.
When budget airlines are actually cheaper
If you're traveling light, on a flexible schedule, on a route the budget carrier flies nonstop, they can be a straightforward win. One personal item, a date you can move, a short hop where a delay wouldn't wreck your day: that's the trip these fares were built for. Take it and don't overthink it.
The savings hold up best on point-to-point routes the big carriers price high. A budget airline that flies your exact pair of cities can undercut a legacy fare by a wide margin, and if you don't need a bag or a seat assignment, the sticker price is close to the real price.
Where the cheap fare stops being cheap
Add a checked bag, a chosen seat, and a carry-on that doesn't fit under the seat, and the gap narrows fast. We got into this in more detail in the hidden cost of a cheap fare: the fees aren't a scam, they're just the part of the bill that shows up after you've already decided. Do the add-on math before you compare, not after.
The bigger risk is the day it goes sideways. Budget carriers often fly a given route once a day, sometimes a few times a week. When that flight cancels, there may not be another one until tomorrow, and a smaller airline has fewer partners to rebook you onto. A legacy carrier running six flights a day on the same pair can put you on the next one. That difference never shows up in the fare and is painfully visible when you're stuck at the gate.
Secondary airports are the other quiet cost. A budget fare into a field an hour outside town can lose its edge once you count the longer drive or the extra transit to get where you were actually going.
How we handle it
We don't pick sides between budget and legacy. When you save a route, we compare the cheapest airlines flying it and send you a few real options side by side, budget carriers included when they win on price. What we won't do is bury the tradeoff. The cheapest sticker in the email is the cheapest sticker, not a promise that it's the cheapest trip.