I would personally be interested in having a copy of the letter. Matilda McClellan is a direct relative, and Felix by marriage. Matilda lived in Limestone County Alabama, and Felix just outside Fayetteville, Tennessee. I do not think the letter is of any other value, although you probably could sell it on Ebay.
I have a lot of McClellan and Buchanan correspondence, and I am interested in the content of the letters as much as anything else.
If you don't mind me asking, where did you get the letter you have, and are there any other letters you know of?