I don't know how new this is but it's news to me, and great news to Mages looking for more Polymorphing fun -- reader Alassiel on Uldaman (thanks!) was poking around Dalaran in the beta, and in the Sisters Sorcerous shop, he found a new Polymorph spell, Black Cat, for... wait for it... 2500g. Yes, it'll cost you more than twice a Gigantique bag, but if you're looking for a new Poly spell and don't want to muck around in the waters of ZG, there you go. And apparently Polymorph: Serpent is also occasionally sold by the same character for the same price.

Both spells seem to be level 60 spells, strangely enough (Update: Because, I learned in the comments, the highest rank of Polymorph is obtained at level 60), so if your 60 Mage has 5000g just sitting around or has a rich alt willing to pay the price, you could be using them then. WoWWiki also says the prices are rep-related, so if you do a little grinding for the Kirin Tor, you might be able to pick them up for cheaper. And don't forget that this is Wrath -- 2500g is likely only a few days of dailies anyway. Polymorph on!