That will probably be the only one for awhile.
Most MMORPG's usually offer deals and slashed prices once people start cancelling subscribtions... a cheaper entry price will get more people back into the game quickly.
Since World of Warcraft is absoutely huge at the moment, I wouldn't expect any other deals for a good long time.
If MMORPG's are your thing, just pay the 50 for it, it's worth every penny and you will DEFINATELY get your money's worth. I put a ton of time in it, at least 200 hours if not more, and I don't regret paying 50 dollars for it at all.