I made my own with Pimoroni (with a small OLED), a Raspberry Pi Zero, a UPS Lite, and a PoE hat (I could switch between these options). Then I updated the OS, Python 2 to 3 and RIP. They sell a Pico version nowadays. It cost me approx 100 EUR with all mats, during Covid pandemic. It worked nice in Prometheus, Grafana, HomeAssistent. Put one outside, too.

After I bricked it I bought two Switchbot Co2 meter (these got e-paper, though I got those also lying around), for 60 EUR each, used some nanotape to attach these on a spot I like. I use one upstairs and one downstairs. If we sleep with all 4 in same bed, during cooking, or bday parties these go high. Otherwise not. Is it needed? Not really. You physically notice the air quality and take action. Except when you don't. My mother in law doesn't notice her fumes in house (she lost some smelling ability) so I bought her a Switchbot, too.

There's no way I would pay more than 200 EUR for such a device. Not even 100 EUR. I get it, you're from Switzerland with a high cost of living, but that isn't my problem.