Ehh. You're probably not going to find very much that's cheap. I finally gave up and bought a 40-tall for $40 + a $10 lid. I really wanted something with front access, but most things that are cost $100 more and most don't have the viewing area I wanted, and also didn't seem very good for a natural-type enclosure. The big driftwood cost $22, and the brown vine was $4. Half a bag of cypress mulch works out to $0.75. Overall, I think I did pretty good.
About humidity - I haven't had any problems yet keeping the humidity up. He had a perfect shed 2 days ago. I just keep a damp towel covering most of the screen, and mist twice a day.
<img width=400 src="http://members.lycos.co.uk/pinatamonkey/atb/boa01.jpg">
Have you checked out
www.corallus.com? Probably the best site for info on tree boas.
Oh - I just checked out cagesbydesign - the 2x2x2 cage is almost $190 - more expensive than boaphile or vision cages.
I don't know what you mean about herpcages.com. There is a yellow box with links in the upper right corner of the page. Links go to cages, racks, accessories, history, show schedule, and order form.