Don’t even have to pay for the base game (which is everything except the current expansion) now. The game is free for the first 20 levels (with social interaction restrictions) and with the rescaling at the end of the last expac meaning you can play those 20 levels in pretty much any part of the world (of warcraft).
The current xpac is a one off cost still, with the sub required to access the end game levels and remove all the social restrictions.