The output on this thing is supposed to be 500-700 ml / hour, but it puts out about as much as the Costco one for 35 dollars. However, having said that, what you are paying for is the Heating element that puts out warm mist and the fancy top loader, controller and digital display. It's worth it if you need a high output warm mist.
It is very convenient to topload although it's a cost thing. Is it worth paying 3 times the amount for a cool mist humidifier? Probably not. For plants though I'd definitely stick with cool mist and would go the cheap-o route.
Also note that because of the heating element and cotton filter, I would only use very low mineral water. It seems it would be a hassle to clean all the time with vinegar. While cool mist machines can virtually run without cleaning and build up calcium.