These motor-generator sets were very nice machines. Smooth, with a very stable arc and good slope control. Not so cheap to run with 220, better with 3 phase, as the larger ones were driven. Minor maintenance required on brushes and bearings as well as internal cleaning from time to time. Limited to SMAW or light carbon arc. I loved to weld with these MG sets. I don't think I have seen one in use for at least 30 years.
The market for a single use machine such as this is very limited, there are few who remember how nice they were, and there can't be many left so very few sales to compare with. Similar arc characteristics to the better known SA 200 with gas motor, for which there is still a niche market because of the excellent arc performance. I think most of the SAE 200s went for industrial scrap as they were considered obsolete.
I don't know what it is worth either, but $500 would seem fair, if the condition is excellent and you can find someone who wants it, which will be the hard part. Just my opinion.
Peter Bumpus