I just got back from a transatlantic out of Ft. Lauderdale, to which I flew on Southwest. It is my airline of choice. I like their prices, the humor of the flight crew, and the fact that they have consistently gotten me AND my baggage on time to where I want to go.
If you buy the preferred seating at the time you buy your ticket, the price is $12. The airline seats passengers in the order that they checked in, with the earliest time to do so being 24 hours prior to flight time. Since window and aisle seats fill up first, someone who checks in late will probably wind up with a middle seat; and those are the people whom Southwest will try to entice to buy a last-minute upgrade for $40. They can usually do that because they reserve the first 15 positions for those who pay the extra fee, and normally no more than 5 have pre-bought it. I've been on two SW flights where no one had shelled out the extra $12; on one of the those, two men decided to pay the $40.