FMS is not peer to peer to this date, therefore you'll always pay for the bandwidth as long as you're not going P2P.
Flash Player 10 has these capabilities built in, but Adobe is yet to provide FMS that is P2P capable, beside the "hosted beta" called Stratus where Adobe makes even more confusion, making it unclear whether you can release a commercial application or not under the hosted Stratus server, rendering all these features useless (these are the "advantages" of proprietary, obscure protocols, close minded companies).
And that's FAR from being all that will stop you from investing your first $$$ in a FMS license! Beside the protocol issues, Adobe doesn't provide an open API Achoustic Echo Canceler within Flash making the audio communication extremely painful, unreliable, specially without headphones, not to mention the network latency causing seconds of audio delays. The AEC is there, but it's only to be used by the Acrobat conference services (same for screen sharing).
I don't know about Java based solutions - I suppose you'll be hit by the poor JRE penetration on PCs nowadays and the long antipathy for applets but that might do better than anything from Adobe in the end!