How much is a John Deere F620 worth?

Not nearly enough info to answer this question. For one, John Deere doesn't use one nomenclature exclusively for one piece of equipment. Also, we don't know the model year, overall condition, what attachments and accessories you have with it and the condition of those, engine hours, etc. You'd be better off calling JOHN DEERe dealership to have someone come and appraise it.