I'm no longer an active multiboxer or WoW player, but when I used to box I shelled out the full $50 for multiplicity. I found it easy to configure and use; it solved a lot of lag problems that some of the other kbd/mouse sharing apps had for me.

Anyhow, you can get Multiplicity Pro for $13.99 this weekend through Stardock's Impulse service. http://www.impulsedriven.com/mpp