Actually, if you download it legally from the official World of Warcraft website, you pay $20US for the first month and $15 afterwards, so you don't really pay much for the actual game, besides like $5.
Unless you go to Walmart or ebGames and spend the full price.
The expansion pack, Burning Crusade costs a lot more. $40US online, not sure on the in store price.
I trust Blizzard with my credit card number. Plus if your state doesn't apply tax to online stuff you'll save the tax that would be applied if you purchased a game card.