There's a little bit of chit chat in this thread on Top Cat values, nothing concrete though.
http://www.oople.com/forums/showthread.php?t=129730
I'd be starting at £150 for a good condition complete and working used roller, the closer to standard and new built the more it should be worth and vice versa.
Of course examples will come up for sale cheaper generally by sellers that don't know what they have or for models that need so much restoration they prove to be uneconomical.