Does having the Secretary of the Treasury's signature make a note valuable, or is it simply graffiti ?
If that's an authentic signature (no reason to believe it's not), I think it would boost the value quite a bit. It looks like a presentation note the owner kept folded up in a wallet or purse. edited
It always goes to the question of what is a value to the collector and the person who may be interested in purchasing the note. I would argue that since this is a signature of an earlier Treasury Secretary on a silver dollar that it would be worth more than face value. Exactly how much? That is debatable. Most of these signatures were gathered for the fun of collecting and to have another interesting piece in the collection.
These are courtesy signature notes. There are quite a few of them on eBay. You can some idea of the value there.
I collect courtesy autographed notes. Humphrey is kinda rare, especially compared to the other name on the note. Ivy Baker Priest was a prolific note signer. I have three of hers but still need a Humphrey autograph. Most I see are in better shape but it still has value.
My very less than expert opinion of your note is this. Without the signature your silver certificate is a typical, not rare, well worn, common note worth possibly $1.25 retail. How can the courtesy signature bring that value down any lower? IMHO the signature can only bring the value up in this specific scenario. I don't know how much higher than face, but it's got to be more than without the signature.
A signature in a bill is damage to the bill. You can't get around that as it doesn't belong there. Now, the bills condition, who's signature and how rare is there signature will determine the value. In the case of Humphrey's signature, thats on the rare side so it will increase the bills value.
The sheet signed by Mary Ellen Withrow is an example of how active she was signing autographs. You often saw her at large coin/currency shows behind a table signing virtually anything you put in front of her. Autograph seekers lined up to visit with her. Many of her notes were submitted for grading. My stash and a photo of Mary Ellen signing notes.