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. http://edwardmallon.wordpress.com/2014/07/01/a-10-diy-data-logger-is-born/
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.