http://www.detecting.org.uk/html/Sup...nd_Frauds.html
"Currently, it is estimated that 1 in 10,000 bills is a super note" Thats 0.01% and there is not as much fake 1$ as there are fakes 100$.
So I guess its still about 1% or more. Dont worry too much about fake bills, if you cant spot them, the guy who will take you order at mcdonalds wont have a clue either.