It's more expensive and doesn't produce as fine of a mist as an equivalent humidifier made for people modified for terrariums.
The thing about a humidifier is how do you plan to go about controlling it? It can get expensive getting an adequate thermostat with a humidity gauge option that can shut it off when the humidity reaches the desired level.
If you really want to do it look into an ultrasonic cool mist humidifier and check youtube for DIY mods to set one up. Now if you are getting a PVC enclosure you are not going to need a humidifier. Again however you will either need to manually turn on and off the humidifier or buy a thermostat that can control humidifiers like herpstat can.
These ultrasonic cool mist ones actually keep dispensing "fog" instead of going "limp" after like 5 minutes. If you do this make sure to get a cool one and not a warm humidifier for obvious reasons
Here's a quick video someone made that shows one of these type of humidifiers modified (ignore his voice lol)
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