Costs for all components tend to be a bell curve. There is sort of a magic point where smaller drives don't get any cheaper. There is another one where the the price for extra size starts to climb. I don't know if you can even get a 10gig drive anymore. I have seen 40gigs for $40 and 120gigs for $90.

Unless you already have it you need some kind of hub as well so the systems can talk to each other.

I'm no expert but when I try to boot my single system if the bios can't find a mouse, keyboard, or display I never even get to windows. You may have to get specific types of motherboards or bios to allow you to run without input and output divices.