It is a huge chunk of two billion dollars. Even assuming he got the full 2 billion (he didn't) and that there wasn't a penny owed to the IRS, he just spent three percent of his entire fortune at once. That may not sound like a whole hell of a lot, but the difference between Notch and other people who could afford to buy this place, those other people probably still have cash coming in. So the man spent 1/30th of his life's fortune in a heartbeat. This is why lottery winners end up in bankruptcy, two billion is a fuck load of money right up until you start spending like it isn't.
When you have the kind of money he does your fortune is your income, he can make 50 million a year with VERY conservative investments.