| Joomla + Community Builder (Joomlapolis) is the cheapest and most extensive system I've tested. - there is a cost to CB for the guide, the software/script is free and open source.
It has a lot of features out of the box with a ton of plugins that allow it to be extended to whatever is required, specially when you can addon even non cb components into Joomla itself.
Social engine seems pretty good too from the demo's. |