I would think that information regarding the amount of counterfiets the casino might get would not be made public anyway. It would almost invite people to bring in more

In regards to $50's, I have been told that they are deemed unlucky and it is pretty rare to find them in the casino in the first place. I've had times that I've asked for them at the cage and more often than not have been told they have none

As far as 20's go, I have to agree that those would be the most likely bad bill to be passed.
On a different note (no pun intended) I recall reading somwhere that more than 09% of all 20's are found to have traces of cocaine on them