ReptileDirect.com sells this air exchange system for $11.99, plus you will need to get a cheap air pump to run it. I bought that a couple of months ago, and turn it on when my bp goes into shed. Its great because it keeps the humidity at about 70% without having any part of the cage being damp at all.

http://www.reptiledirect.com/index.a...ROD&ProdID=437