There is actually an open source version of Community Server. They must have forked the project a long time ago, but you can find it at
Community (Advanced) Starter Kit (C# .NET) w/ AJAX - Home.
I think Community Server is a great product, but the license fees are killer. It is hard to justify spending that much money. The open source version is really coming along and I hope will give the commercial version a run for their money.