I have no idea. That is something that would need to be appraised by a professional. I don’t collect anything besides my piles and piles of books, so I’m the wrong guy to ask.
I would imagine that any letter from Lincoln would be worth quite a bit of money, and communications from Lincoln to his commanding general would be even more valuable. That particular letter is perhaps the most famous communication between Lincoln and General Hooker (that’s the “What I now ask of you us military success, and I will risk the dictatorship” letter), so it’s a very valuable letter.
Like I said, I wouldn’t know how to appraise something like that, but I know it would be extraordinarily valuable.