In the US, budget airlines tickets cost about 10-20% less than regular airlines tickets and in Europe, budget airline tickets cost about 30-50% less than regular airline tickets.
How do they do it and make money at the same time?
Budget airlines choose to fly to cheaper airports, and they usually dominate these airports and that enables them to negotiate cheaper fees.
No first class:
This allows the budget airlines to add as many seats as possible and reduces the staff needed to cater for first class passengers.
Cheaper cabin crew:
Budget airlines normally employ cabin crew at the beginning of their career as well as minimizing training. Multitasking is expected from cabin crew and saves up to 4 cabin crew members per flight.
Everything inflight is for sale:
Drinks and about everything else will be paid for during the flight. Ryanair is famous for selling lottery tickets. Anything to increase revenue.
No additional systems such as TVs, reclining seats or seat-pockets. This saves costs by initially buying and maintaining such extras. No seat-pockets save time when it is time for cleaning the aircraft for the next flight.
Buying in bulk:
More money is saved by buying aircrafts in bulk like Ryanair buying 151 Boeing 737s
Usually, budget airlines have the youngest fleets and this means the most fuel-efficient aircrafts, saving them even more money.
One type of aircraft:
Budget airlines usually operate the same type of aircraft like a fleet of Boeing 737s or Airbus A320s. This saves costs in terms of training and time. Time is a very important currency for a budget airline.
Route planning is extremely important and below is an example of routes that are flown by a budget airline in a day:
Brussels to Copenhagen
Copenhagen to Brussels
Brussels to Prague
Prague to Brussels
Brussels to Nimes
Nimes to Brussels
Brussels to Treviso
Treviso to Brussels
Again, time is very important and the aircraft needs to be on the next route in 30-45 minutes.