Nobody pays that much money for a game, but that also isn't what MS is buying.
MS is purchasing access to the users of that game, the community, and the ability to influence them. It's the same reason why WhatsApp sold for 16 billion. WhatsApp itself is just a poorly built IM service, really just a dime-a-dozen app, and not even the best of them. Anybody could develop something like it for less than 50'000 per platform, however, through timing and luck, it managed to become the most popular, and it's vast user base is what was sold. Minecraft is no different.