Product review (4 stars): You can get this part a lot cheaper under another name.
Just to let people know that you can find this same part for a lot less on The Filter Source.  It's the same part as is used for the Bionaire BU5000 Humidifier Replacement Part: Water Tank Cap 7921000024806, and for the Homes HM5700 (which is what I needed it for, and a whole lot of other warm mist humidifiers.  It costs $4.50 plus 5.95 shipping.  With my MA tax it cost $10.73 total.  Good luck!