From what I have seen in actual auctions on Ebay for the same FDC I am not sure I would agree entirely with the valuation of $50 to $150. Here is a current auction on Ebay for the same cover with the same stamp, cancel, and cachet (these cachets were made by Linprint) and in much better and undamaged condition (the rough opening on yours is a detraction in potential value to a collector) http://www.ebay.com/itm/U-S-735a-19...AOSwopRYY046
This is a completed auction and no one bid the starting bid of $5.69 and so it did not sell. I notice that it is again up for auction and so far there are still no bids. Or here is a completed auction that did not sell with the same cacheted cover only this one has the full mini-sheet of stamps and is in excellent condition - no one offered the starting bid on this auction of $16.95. http://www.ebay.com/itm/U-S-735a-19...AOSwopRYY046
The value to you as something from your father is priceless and you should treasure it. On the open commercial market, the damage from the rough opening plus the soiling plus it having only a single stamp would bring the value down, but it being addressed to someone in a CCC camp would add some value. My thought is perhaps more in the $5 to $10 range if you found someone who wanted it for its CCC connection rather than its value as a cover.