On Thursday, April 15th Blizzard released the first in-game mount to be available for real money.
The Celestial Steed can be purchased from the Blizzard Store (US) for $25 USD, or from the Blizzard Store (EU) for 20€ or 17£. As Blizzard is quick to point out on the store:
Once activated, this World of Warcraft in-game mount key applies to all present and future characters on a single North American/European World of Warcraft license.
In addition, the speed of your Celestial Speed will scale with your Riding skill, all the way up to +310% — but only if you already own a +310% speed mount.