Here's one for $AUD165:
http://www.torstenweller.com/rDetai...5008&this=it
Now, yours is not as valuable as the one from Torsten because yours is dealer-to-dealer corrspondence and you must assume that the sender was aware of how he was constructing the postage payment. The one for sale by Torsten is true commercial correspondence where the sender was just sending a letter without any regard for the importance of the stamp affixed. Torsten is also not scared to price his material at high prices.
Torsten's is also a July 1912 date.
Your cover may be closer to $40 on that basis but that would be about it.
You may care to offer it to Torsten to see what he would pay you for it. You could reasonably expect him to pay 50% of what he would sell it for.