$75 a month for 5 accounts = $900 a year. 5x $20 for vanilla WoW = $100. 5x $30 for TBC = $150. 5x $40 for WoLK = $200. That's $1350 for one yearl of multiboxing (although you do get some free time for referring yourself). A $20 one-time fee doesn't seem too much. Multiboxing isn't a cheap hobby.

Of course you can always just toon B on /follow and swap back and forth to a 2nd keyboard and mouse whenever you need to do something :P But yes there are a few "free" ways to do it, they just aren't as simple and quick to set up as some of the paid for software that;s out there. Really depends what you want to do.