These fantastic Beatles discoveries are becoming too frequent. Sgt Peppers Drum on album cover found in a barn, the queen letter at yard sale....
More likely someone close to Lennon purloined this letter, needed the money, but did not want to be associated with it so arranged for a friend to "find it" at yard sale.
I am not sure that this is all that fantastic as it was never actually sent.
If we were talking about a letter with a Buckingham Palace receipt stamp then we might be in the $50,000+ league but for me this one would be more like $10,000 - $20,000.
Obviously it is much more interesting than a run-of-the-mill signature given that it is on John's letterhead paper and is apparently a draft of a letter to the Queen but I wouldn't get too excited about it.