Value of a used one? Hard to say. They are super expensive new, maybe $1600-1800. Used, 1/2 that?
Very cool though. A fab shop I worked at had the Victor (?) version, it was before the widespread use of plasma and/or "cheap" and easy CNC. Guys could cut out heavy plate that looked like it was laser cut with that thing. Real impressive in the hands of an experienced welder.
Hard to give a price since they aren't very common but I'd guess 4 or $500. I don't know what a new one costs but if the price was too high, people would just get a radiagraph instead.