This is from my experience as a server programmer for a commercial MMO:
MMO middleware is not a well established commercial field, and there have been very few successful attempts to use mmo-specific middleware, at least within the realm of large scale MMOs such as your World of Warcraft.
Multiverse has been used in a few smaller MMOs and I believe is free for noncommercial MMOs. However, the graphics are somewhat dated and it is not designed for large scale deployment.
Hero Engine has been around for several years, and has some very good content creation tools. I have no direct knowledge but have heard disappointing information about the usefulness of the back end server package, and I am yet to see a shipped game using this middleware. The upcoming The Old Republic MMO is using this engine, although I do not know to what extent.
BigWorld has been integrated into several actually shipped medium-scale MMOs so has a bit of a leg up. I again have no personal knowledge other than being impressed by their content creation tool at GDC. I suspect they are not particularly cheap.
In general, I would be extremely skeptical of using any Middleware solution that has not been used in actual shipping product. There is a huge gap between making content creation tools and making a scalable, working server architecture. Until an MMO has gone public and been hit with real users there's no way of knowing if the tech is good.