Placing values on covers can be rather difficult, as no two covers are exactly the same and a lot depends on the individual item, its condition and desirability, and any history that can be found as to the addressee, etc. Then it becomes a case of supply and demand that determines the true worth of the item.
For starters, here's an
ebay listing of a similar stampless cover from Dansville, NY dated 1840. Although the seller is offering it at a BIN price that was $40, discounted by 33% to a new price of $30.15, even at that price "reduction", no bids have been received, so whether it will ever be sold anywhere near that price is highly questionable.
http://www.ebay.com/itm/DANSVILLE-N...em4608380145