Today we learned that airlines view WiFi as integral to the whole "flight" experience as the very wings on the airplane itself, but it's one thing to hear about it from an industry perspective, and another to see that sort of manic desire for Internet in action. So watch United Airlines take this whole plane apart for it.
Installing WiFi on a plane isn't as simple as plugging in a router to the outlet and an ethernet cable and calling it a day. In fact, it's much, much more involved than that. When airlines want to install wireless Internet in a metal tube that goes blasting through the sky at over 400 miles an hour, they've got to take the whole thing apart. Out come the seats, the ceiling, in goes bits of plastic and a whole bunch of people to wire the whole thing up.
In short, yes, you probably have to pay for it, but you're getting a hell of a lot of labor for your money.