Esty,
You could be right, but, if this letter is a fake, then they've really pulled the wool over the eyes of two of the most "reputable" of the antiquarian book dealers/jobbers.
Simon Finch is a fancy place with wonderful things but which many people feel is overpriced. (They have a first edition of the Hobbit currently listed at 24,000 pounds.) Hence the crazy price tag on this letter. Abe Books is also very well known.