Running a server with tiny handfuls of people doesn't really cost anything.
I love single player, but I agree that Minecraft really shines when played with other people.
Theoretically, I could run my PC as a small server and it should have enough power and bandwidth to allow 4 or 5 people on at a time. In reality I couldn't do that as I'd have to leave it on all the time and my PC has an overheating problem.
In Minecraft news, Notch has included cake. There's also going to be an update on Thursday with some fun new content.