What are the costs of 1 hour flight in modern low-cost airlines? Thinking about "low cost" airlines that often tickets of about 40 Euros for 2 flight hours, I was wondering how these companies can cover their costs.

Considering for example EasyJet that fly with Airbus A319 (150 seats), with a unique cost of about 90 million dollars, on a typical 2 hours flight (common average European route) every flight could gain about 6000 Euros in tickets. 

I can't understand where is the "gain" considering fuel, crew costs, maintenance etc...

Can you give some references?
For the low-cost model there's an article called The true cost of flying revealed, which uses a 154-passenger A320 for its figures.


  According to the calculations, each passenger on board an Airbus A320, which has a capacity of 154, costs the airline $68.50 (£47.06) for the 260-mile journey.
  
  The one-hour flight costs the airline an estimated $2.50 per passenger [for fuel].


The total cost equals \$10,549 per hour "with profits as low as \$10 [per passenger] on some flights".

Also from the article—


  If you want to be a millionaire, start with a billion dollars and launch a new airline.— Sir Richard Branson