There is a chance that there is a makers mark somewhere on that lock It might be under the rust. Take a CLOSE look. This would help in finding a value. There may have been several makers of this type of lock even if all of us may not have seen them. By knowing the maker we might be able to find a catalog with this model of padlock. Also give it some time for other members to come on and comment if they know something. Not everyone logs in everyday like I do.
As to the value.. it is a hard thing to pin down unless you have other sales to compair it to. It is what ever the other person offers is what it is valued by that person.