Without seeing a good scan of the cover and condition of the cover it is very difficult to determine a value. With covers so many different facets can affect the value such as postmarks or auxiliary markings, sender or recipient, destination, etc. eBay
is as good an avenue as any but with covers pinpointing a cover value is difficult. 2 people might see the cover and 1 would value it at $20 and the other could see paying $100.