It seems like most on of the folks on this list already know more than I do about building data loggers, so please forgive me if this amounts to spamming the thread. But I wanted to post a link to a really inexpensive data logger I have come up with for one of my projects, based on a 3.3v Pro Mini and a cheap DS3231 breakout board from eBay.
I have put a few of these together now, and they all seem to be running well, with the only caveat being that the voltage regulator on the pro mini draws about 1.5mA* while the unit is sleeping. So not great for long term deployments, but good enough for the bench-top prototypes I have used them for thus far. Also, all the usual caveats apply regarding the reliability (drift, etc) of those knock-off RTC’s. Rest assured that I use more reliable clocks for actual fieldwork. Still, you can put one of these things together for about $10-$20 depending on how cheap you want to go on the parts, and that’s handy when you are just trying out a new sensor.
*Edit: Read through the thread: My current builds are usually around 0.35 mA. Without knowing it, I had bad SD cards drawing alot of current in the first few, and I did not pullup the unused lines (8&9) on the SD cards, which also caused some cards to draw more power because they did not go to sleep properly.