They purchased mojang. That usually includes the cash on hand and monetary assets. Recent estimates I've seen were talking of 1.5B in tangible assets for Mojang. Thus, it's a 1B premium, meaning that they value the profits per year for the following years at 150-160M, which is very easy to reach with that license.
Minecraft has sold more than 100 million copies, every of them having the possibility for paying for a minecraft realm monthly to share their creations. A lot more people can buy the game. Then, if it start fading away, they will be able to grab a lot of money with minecraft 2.
Finally, they can also use this license to push their devices for which there was no minecraft before. Optimized for Windows Phone!@
This is not an insane amount at all, compared to the twitch buyout or such..