I would like to note, that though you do pay for the CD's, each CD comes with a free month, so if you play your cards right, you can get them as you need them (the original free month will all but cover the whole game) and if you get the refer a friend, then you get an additional 2 weeks or so... then wehn you start getting close to 58 (or if you want to play a Blood elf or Draeni (sp?)) then you can get the first expansion which should be dropping in price... as you get closer to 68, then get the third, that way you aren't fronting all the money, and you have time to let the prices drop a bit and take full advantage of the free months...
That being said, Harry is right, you pretty much sell your soul to do anything decent in the game (raiding, arenas, or high end PVP)... I sold my account last April, and I was a part of a top tier raiding guild, so I do speak from experience... if you want to start playing, make peace with rarely seeing daylight again.