Burger King at the airport, for example, has the exact same prices as the city, and they actively encourage people to spend little money.
There is free water, a reasonably priced airport hotel, and even free showers at international gates. There are plenty of free seating (so you don't have to go to a cafe), free WiFi throughout the airport, and a few convenience stores scattered around.
It's not that the flight tickets are expensive either. The airport tax is less than $5, and tickets are super low cost for all Air Asia destinations.
The high rent, airport cut, human costs, etc can make things more expensive, but don't forget the stores charging 2x the price just because they can. What are you gonna do? Go back through immigration and security again? I dont think so.
It's Air Asia's main airport, and Air Asia is quite good at managing costs. Their flights are shorter (only in south east Asia, with just a few connections to China, India, Korea, Sri Lanka, and Nepal), so there is no need for a bigger airport hotel.