I recently ran across the temperature logger tutorial posted a while ago by jennajane, and wanted to chime in. First of all way to go! I started my path to learning Arduinos with the same Adafruit logger shield not very long ago, and it's been an entertaining hobby for me ever since. But I have been plugging away at it for a while now, and I just posted a DIY plan for a very inexpensive ($10-15) logger based on the sparkfun Pro Mini boards. Also there is a waterproof housing I made from PVC pipe with only one hack-saw cut & some PVC adhesive for about $10. The housing is made from 3" pipe, because back when I started, I too was working with the Adafruit loggershield, and a Uno board, which will just fit into 3" pvc pipe if you slide it in vertically.
So if you are feeling adventurous with the soldering iron, you can grab the wiring diagram, and the script to run the datalogger at:
It is made from only three components, and there are links in the post to sources where you can buy the parts cheaply from ebay. I have already been using the DS18B20's temp sensors (because they are about $2 each, and will read down to 0.02 deg C ), so the code I posted on git hub already has support for that sensor if you are trying to track temperatures in your greenhouse/barn etc. If you want to add that sensor alone to your project you can see my development work on it at:
Anyway, I hope something in there helps anyone hoping to build a really affordable data logger for their farm.