I'm not an expert on Ms. Harry's signature although it looks good to my eyes. With that said, it's a nice piece of Blondie early history. Would be really nice if you had the shirt to go with it. I would say that this is probably worth in the $200 range. Of course, in an auction if you can get two people who want it a lot, the price could go higher.
I agree that the letter needs to be matted and would look great. I still stick with my original estimate although an eye popping display would put it in the $250-$300 range. JMO.