From what I understand, each update is a standalone game. And you have to pay 8$ for each one. If you sub for 8$ a month, each update is included.
It sounds bad, to me at first. But if you think about it, you either sub for support and download each one, or just pay for one version, and if you don't want to pay monthly, you can just wait a few versions until you feel a sufficient amount of content is added.