Notes with serial numbers made up of identical digits are rare, with only one in 11 million bills printed boasting this type of detail. As a result, they're worth big bucks. Considered lucky by some, this dollar bill, which has a serial number entirely comprised of sevens, sold recently on eBay for $600. Even more impressively, a 2006 $100 bill with a ones-only serial number was available recently on the CoolSerialNumbers site for $4,500.