Sanders Autograph Price Guide has Helen Keller Typed Letters, Signed at $765, but there are lots of variables. Join the History group and upload it there for more input:

http://live.autographmagazine.com/group/history

You should get action a little faster. Good luck!

Thanks for the details, Don. Being signed in pencil, it couldn't be a signature printed with the letter. I don't know if she sent those out for fundraising, but it came to mind. 

Are you looking for feedback on it or were you simply sharing your favorite autograph?