SilverDon said it best. It's worth what someone will pay. On occasion I buy doubled coins but they have to stand out to me. In general people are selling them on
ebay for about 150 bucks in a sealed set from various years. Considering there's a buy it now option I'd pay a small premium over issue price depending on scarcity and no more. Probably why their still for sale too! A single coin in circulated condition to me, No more than a few bucks over face if it's a nice machine double in the right place.