A new WoW account with first month costs 40-50 dollars, no getting arround it, no matter whether you have the game already or not. A car can be bought used, a WoW account cannot, a car can have many drivers, a WoW account cannot.
You're paying for the account when you buy the game.
Look at it this way. When you buy the box, it comes with a CD-key. You use that to make an account. The account is what you are paying for, and you get a month along with it. The account costs 50 bucks, the month costs nothing.
When you buy a car you get a tank of gas. The car costs however much, the gas is free.