It's value is in how the machine runs and how nice it is to work with it. I would be surprised if you could get more than $75 for it, with cabinet, sold locally. On e-bay, in better economic times than these, you might get more. If it needs any maintenance, wiring work, or cleaning, it is worth it, IMHO. Seldom will you find a more reliable machine or a better sewer.
However, for it's engineering and the smoothness with which it runs, and the joy it gives by using it, that machine is priceless, IMHO. Be nice to it, keep it oiled and cleaned, and your great grandchildren will use it.
Cricket