Big Mo tractors are "industrial" versions of a jet star more or less. As with most vintage industrial tractors, it is hard to find a nice example that has not been beat to hell. Most have seen loader or backhoe service.
There is a cemetery not too far from me that still uses a Big Mo 500 backhoe for digging graves. It has around 8000 hours and is still going strong.
Most "beaters" seem to go for $1,500-$2,500 depending on location. I sold my Big Mo 400 a few years back for $3500. It had a 3pt hitch with newer paint and tires.
Hope that gives you a general idea. Take care, Art.