Product review (3 stars): Super thin.  Not very efficient.  Good in a pinch.
These are definitely the cheapest filters you will find.  I have never found anything close to this price...

BUT...these are SUPER thin.  This means that even at the highest settings, my humidifier will barely empty an entire tank over the course of the day.  Are humidifiers are only 1 gallon tanks, so this translates to very little moisture in the air compared with other filters that I have used that are generally about twice as efficient.  These are good to have around as backups, but I would only use them in a pinch.