The benefits of screen enclosures, especially with front access, far outweigh the downfalls. As far as the humidity goes, there are many ways to help keep it up. Have a potted pothos or philodendron in the cage. The plants help keep the humidity up, and the large leaves provide excellent hiding places. Also, if you are very concerned with keeping humidity levels up, check out the Chameleon forum. There you can find ideas on using an ultrasonic humidifier or coolmist humidifier and some PVC pipe, which allows you to easily "pump" mist into the screen cages. Set the humidifier on a digital timer, and you are set.
Depending on how many cresteds you have per tank, I would recommend going to reptiledirect.com, where they have smaller screen enclsures (18"x12"x20"
made by ESU for $30 (the same ones in Petco for $45). Reptile depot also makes a "Crested Enclosure" that has some sides of screen, and others of acrylic to help increase humidity while allowing cross-ventilation. However, I have had problems with Reptile Depot before ordering online - if you see them at a show and can get them - go for it. If not, you may wait a while before you receive your items. Another cheaper option are the Reptariums, but these are my least favorite as crickets can eat through the mesh; the mesh is very dark and hard to see through, and they can be a hassle to open and close with time.