It is 2/3 of the cost of the unit, but easily replaceable. The thing that I like most about the ultra sonic foggers is the lack of heat generation. When it is 90 degrees outside, I am looking for an excuse to stand in front of the unit, with minimal clothing...The reason I ask is because the Hydrofogger doesn't appear to use ultrasonic discs, which seem to be the most expensive maintenance needed for these devices.