These aren't that expensive in bad grades but yours is beautiful.
I think for the American market grading helps if you browse
ebay there are Black Eagles reaching some big money in good grades. I know signature combinations also have a bearing...
I have a book on order that would have let me give you a bit more info on that but it's been ages in the post.
Anyway it's a good note if you want to know its value don't take anyones word for it go on the bay and see what you would have to pay for something similar.