LambdaMOO server can be run on a $5 EC2 instance without much problems. The database resides in memory though, so if you have a giant game (like HellMOO database from what I've heard) it can take like a gig of ram. However, with careful management you can run a game with 100 people online, in a massive world, with tons of code and rooms and such, with 250megs of ram.