The War Z had a rough launch on Steam and it was ultimately taken down, to the point where Valve even issued refunds for it. The developers apologized for making everyone upset, and after a couple of months, The War Z has found its way back onto Steam.
$14.99 will get you the base game, and there's also a $24.99 and a $49.99 edition that offer you large chunks of in-game currency to spend. According to the release notice, Valve worked with the developers to address issues the community had with the game. If you already owned the The War Z and didn't return it, you will get this new updated version for free.