Well, afraid I'm no help in Thermometry.
Though, if you're looking for the cheapest Arduino, you can purchase the Atmega8, 168(etc), flash them on your board, then just pop them out, solder it up / hook up to breadboard and you're good to go!
For a standalone Arduino, you're going to need to either set the internal clock(I believe this requires some code change), or get an external 16 MHZ crystal.
http://itp.nyu.edu/physcomp/Tutorials/ArduinoBreadboard
Also, here is one of my favorite Arduino help-sites.http://freeduino.orgAlmost everything you'd ever need! 
Price wise, standalone Arduino is your best bet, as you can get Atmega8 for $2-3 each on Ebay. You will need to do a search on putting the Arduino boot loader. (Or any other you're comfortable with)
Also, take a look here, Todbot post using just an Atmega8 and very few components for a standalone Arduino!
http://todbot.com/blog/2009/05/26/minimal-arduino-with-atmega8/
Hope this helps.