I don't think you overpaid. If you look at notes that recently sold for under $200, they all appear to be in slightly lesser condition than your note. Given two identical bills, I'd choose the one without the stamp...but we don't get that choice in real life.
Take a look at sold notes on
ebay that went for under $200 and see if there are any you'd want to swap this note for. If not, you know you got a fair deal.