Awesome thread to find! I've been keeping an eye on AirPi, but I'm concerned about the soldering requirements, bugs, and lack of calibration.
I did a bunch of research today on alternatives, and it looks like Netatmo is the current best, at $145 for CSV export and the critical sensors (temperature, humidity, CO2) plus pressure, sound, and IFTTT support.
Foobot would be second, since it has more sensors (temperature, humidity, CO2, CO, tVOC, PM2.5) for $199... Unfortunately, it doesn't have any export, nor an API, nor IFTTT support.
Is there anything on the market that's better than the Netatmo, and still under $200? (Everything else from @gwern's post on 2014/9/29 is either unavailable or more expensive.)