Too many unknowns to make any kind of estimate Mattyman. Take and post photos. Learn whether it has a title (no title, no value), need to know condition of the paint and decals - whether original or rattlecan, does the engine kick over - with compression, no bad noises. General condition of wheels, tires, frame, body and mechanical parts. Miles on odometer. Missing parts? Standard or Special model.
With that information, you might get some useful help here. Also, check the selling price of similar XS650's on eBay "Completed Auctions".
Bill