For people who do not currently play World of Warcraft, and/or aren't keeping up to date with all the details, the upcoming "Cataclysm" can be somewhat confusing. What does the Cataclsym mean for you if you don't have a current WoW account, but would like to either try the game for the first time, or come back after a long absence? What exactly do you need to pay?
The confusion stems mainly from people associating the name of the new expansion "Cataclysm", with the changes to the world of Azeroth. And that isn't correct. The changes to the world of Azeroth, regarding the zones from level 1 to 60, are called "The Shattering", and are technically NOT part of the Cataclysm expansion. The Shattering happens in a patch 4.0.3a today on the US servers, and tomorrow on the European servers. The Shattering changes the look of all the level 1 to 60 zones, their quests, and also changes some other things, like the possible race/class combinations, and the healer talent trees. All of this is completely free, and requires only the basic World of Warcraft game and a subscription.
"Cataclysm" is an expansion. I costs about $40, and requires both of the previous expansions Burning Crusade and Wrath of the Lich King. It has some additional low-level content, namely the two new races goblin and worgen, with their starting zones. But most of it is content level 81 to 85 (which is why you can't run Cataclysm without having the expansions that allow you to level from 60 to 80), with new zones, dungeons, and raids.
If you have never played World of Warcraft and want now to play a goblin or worgen, or if you want to play WoW up to level 85 and raid with people at some point, you will need to buy not only the expansion, but also the basic game and the two previous expansions. The good news is that those are on sale until November 29th: World of Warcraft for $5, The Burning Crusade for $5, and Wrath of the Lich King for $10. Add Cataclysm for $40 and the whole bundle will cost you $60. Plus of course a subscription for about $15 per month (but I assume the $5 WoW basic pack comes with one free month).
If you played World of Warcraft previously, and just want to see how Azeroth changed, without leveling past 60 or playing a goblin or worgen, you only need to renew your subscription, as "The Shattering" is free for everybody without the need of any expansion.
Finally a personal comment: World of Warcraft is a huge game, with thousands of hours of content. Thus the cost of the monthly subscription will quickly be much more than the cost of the game itself. If you don't have The Burning Crusade and/or Wrath of the Lich King yet, the current sale for these two costs just the equivalent of one month subscription. That is well worth it, whether you then decide to buy Cataclysm as well or not. You pay the same subscription with or without expansions, so playing the game without expansions is the less good deal.
FTC Disclaimer: I do not receive any money or other considerations from Blizzard. Never have, except for a press pass to a Blizzard convention once in 2008. It is actually possible to recommend World of Warcraft without being paid for it.