The only real barometer is what they go for on ebay, (when one actually shows up) and the prices from one of the few LD vendors who specialize in selling them.
http://home.q03.itscom.net/nsa/PioneerHLD-X9.htm
This is the same fellow who sold me the Japanese THX-1138 laserdisc. I inquired about possibly getting my hands on an X0, and sticker shock ensued. ;)