actually, i saw a guy (some professor at a college) on the internet who explained how he built his own. it came out to $300 (i think). I'll try to find the link and post it in a bit.
these things don't really need 8 gyros, except for redundancy and safety.
edit: i'm not sure it's the same link, but here you go:
http://www.tlb.org/scooter.html