Yea i've seen some really run down computers online for $100 or less, I have a friend who can basically take it apart and put it back together so that its clean and way more efficient. Hell my BRAND NEW Windows 8 computer with 6 gigs of ram cost $300. I can run 10 bots on it with minimal lag.Let us know your results, it looks like you've have handled pretty well. Your post gave me the idea of using 'barebone' laptops to run the bots, so something with a very small hard drive, poor quality screen, no cd drive, just like ram, motherboard, processor, screen, keyboard, don't even need to plastic case and setting up rooms of them, you could probably build a basic one for ~$600 and run 20+ bots on it.
I've already started with 1 character and just mining Coal at the low level of 30 already has gotten me more gold per hour then lobster fishing ever did.
If you try this to post your results!