Currently the only way to change your gender in World of Warcraft is to buy an Appearance Change from the Blizzard Store, which costs £13.
Meanwhile, many other aspects of physical appearance can be changed for free using an in-game barber shop. You can change your hairstyle, colour and facial features, even your entire face.
This means pretty much the only thing you cannot do for free is change gender. But that’s, Blizzard has revealed to me, going to change.