A scan would help answer your question so that we can see the condition of the note. However this is such a rarity that there is no established book or "trends" value for it. One way to estimate its value would be to find out if similar "autographed" notes have been sold in the past and how much they went for.
I will hazard a guess of $500 if the note is in F/VF condition. Just my opinion however - other members might have more information and may be able to give you a more informed opinion.
