How much is this Adolf Hitler letter, with signature, worth on the market, I know of no other example?

Well I don't think it is worth much, because it could be a forgery. Do you realize that in the 1980s, "Stern", a major German magazine, printed what they believed to be Hitler's diary? It turned out that these diaries were a forgery. Pretty embaring for "Stern", and I'd say people have become a little careful about Hitler stuff.