Steve I've located a cheaper temp/humidity sensor than the one I used on my dew heater controller ($15 US rather than $46 US). Slightly less accurate but in the same series
http://www.techtoys.com.hk/Components/SHT1x/SHT1x.htm
I have one on order to try out. The one I've already used was the SHT15
http://www.sparkfun.com/commerce/pro...oducts_id=8257
Hooked up to a small micro (I use an Arduino) you can do dew point calcs. I use the dew point calcs to manage the heating cycle on the dew controller but it might also be useful to manage cooling. The code I use was posted some time ago on the PWM Dew heater control thread.
I'm in the process of doing a rebuild of my dew controller so have been looking at options to reduce the cost/board complexity and will hopefully be ready in the near future to post more on that.
Bob