Originally Posted by The Lev

I'm pretty sure that the operating costs of a 737NG are lower than for a 320 - that is why traditionally LCC's opted for 737's - until Airbus started giving them away to gain share. The 737 is a much lighter aircraft.

In addition, WS's fleet age is probably ~5 years, whereas many of AC's A320's are 20 years old, so they would have much higher maintenance costs. AFAIK airlines do not accrue for maitenance - they expense as incurred.