mcc, It seems to depend on area & demand, in general I see them going for $10,000 & up..... when I see one advertized locally. When I've looked on E-bay they seem to go in the $6000 to $12000 range.
I paid $5700 CDN for mine over 10 years ago & it was beat, I've put a lot of time into repairs on the old slave, but found that even the parts are not that cheap any more, so repairs soon add up even if your doing the work yourself.
One draw back of the B series & earlier machines is they are of unitized construction like a farm tractor, so you need to split the machine for major repairs.... like to the shuttle trans, if you can even find parts for those shuttles is even questionable. I pearsonally would try to find a later machine, I think they went to component style construction with the C or D series