We don't know any details. That's all there is to say on that. So any speculation about price, functionality, etc... it's all just that. Speculation.
We do know, however, that this is the first mount being sold at the store; and the first useful in-game item you can directly buy with real world money (if you don't count the mounts from the WoW TCG game).
Video of the steed after the break.