Humidity sensors are typically more expensive than temperature sensors and are more complex to design/build a circuit around. As the price goes up, the fewer additional parts are needed but once the price gets beyond the $50 range, I'm not that interested. For about fifteen dollars and about five more in additional parts, a decent humidity sensor can be built. I plan to do some mapping in my home but I am guessing that there are probably not pockets of high or low humidity in my house and having 2-3 max humidity sensors will be plenty.
A few weeks ago I was reading and article in MAKE magazine about a guys who wanted to lower his clothes dryer electricity bill so he sought to use hot air from outside or the attic. I thought it would be a cool idea if you lived in a place with dry heat but don't think it would be feasible here in Florida. At some point I might try it just to see if really hot humid air could successfully contribute to lowering drying time. I think the key is to find a way to take the water out of the air quickly.