One of the forums I belong to (not sewing machines) doesn't even allow appraisals just because of the wide variation in prices due to differences in region, condition etc. It's actually not a bad thing. It keeps people's feelings from being hurt. Let's say you bought it based on someone's advice for $500 only to see another one for sale a week later for $50. You might be a bit angry at yourself and the person you listened to thinking you over-paid. The best thing you can do if you plan to buy more machines is to just watch Craig's List , Ebay etc, until you have a good idea what prices are in your area.
It's a beautiful machine and cabinet. Furniture of that quality really isn't being made today unless it's custom. The missing slide plate does affect the price though and might be worth using as a negotiating point. Also make sure the shuttle and bobbin are there and that the treadle is complete. Parts can take a while to find and can cost more than an entire machine.
Rodney