This is spammy and $239 is stupidly expensive for a co2 monitor. You can get them under $100 all-up. An NDIR sensor component all by itself is $60 from Adafruit which is a hobbyist electronics place that I like but which tends to be on the expensive side. By that, I mean that the $100 retail units are doing fine in terms of markups. You can also estimate CO2 using a much less expensive ($10) VOC (volatile organic compound) sensor.

Anyway I'm planning to DIY a monitor using the Adafruit module, so it will also be able to log to a raspberry pi and that sort of thing. I decided that approach would have more flexibility than buying a complete unit.