I'm pretty sure you get a free month when you buy the retail version. I once saw a battered up box of the game + TBC + free month. It was discounted to €15, which is just a bit more than a month.
Of course I was clean and didn't buy it.
Which reminds me... if you do the free trial now, you can see how they changed everything for Cataclysm, right? And new race/class combinations are also included in the original game. I think.
