I have an Arachnid "Super 6" Plus II available in Dallas, TX. It is missing a couple of parts, but they are easy (and cheap) to find on eBay. $100, and you arrange shipping/pickup. Even with shipping, that will be well below your $500 limit.
It needs the monitor power supply board (attaches to the top of the main powers supply). The last one I saw went unsold on eBay for $9.99. And it needs the swittch matrix (eBay prices from $0.01 to $30).