Since we already have a massive Minecraft casting community, I think it's a logical place for him to do it: http://twitch.tv/directory/Minecraft
Enter "mojang" there to see the cost.
The cost estimator basically says the incremental cost is $0.27/hour. That's pretty expensive for what is, in this case, basically bare streaming infrastructure (livestream adds some value, but not much for this particular use case).
The essential functionality could be easily replicated on e.g. Amazon EC2 at less than half, maybe even 1/3rd, the incremental cost even as a one-off thing.
Even now, as a not-particularly-wealthy person, I'm often willing to pay extra for convenience, but I just don't see ever being willing to pay 3x the cost of doing it myself for a non-mission-critical application, especially when there are even cheaper options like twitch.
Apparently he had racked up 17,000 viewer hours so far and said the cost was getting way out of hand. I'm trying to see how much that is going to cost him.
It would be cool if we could get Livestream to sponsor him perhaps as a way to promote their service?