Very nice! It's very rare to see this on a modern proof, especially a commemorative. It may have been one of the last few strikes on the last die, and they didn't have another one prepared.
I bought the coin last year. The dealer I bought it from said it would take 25 to 30 years to be worth anything over $100. I paid $25 for it, as I like to hoard away proof mint error's.
Well with the error it just ruins the whole coin I'll take the hit and give you $30.00 for it. It's a nice coin with a very unique error congrats. Ice
I tend to think that there are error collectors out there that may disagree with your dealer! Nice Proof error!