One thing you should really note is Commercial can only support up to 4 multiplayers at one time. Meaning, four computers can be used to join a multiplayer session. If you have 8 multiplayer users with 4 computers, you can have two controller pads connected per computer and program a split-screen, multiplayer game from there.

If the only feature you really want is unlimited multiplayer, you will need to cough up $900 for Professional, even though you want just features from Extra and no features from Commercial -- that is, if you don't care about shaders, which is great if you want to support older and newer computers alike.