The cheapest one that is not eCO2 (estimated) will do the job of telling you when you’ve not ventilated enough just fine. No need to spend hundreds to thousands

The cheapest one that is not eCO2 (estimated) will do the job of telling you when you’ve not ventilated enough just fine. No need to spend hundreds to thousands on lab-grade tech. +- 50ppm, or even 100 or 200 is nothing. You will mainly care if the ppm is around 500 or around 1000-2000 (depending on preference). The devices that have alarms are quite nice. Leave them on a desk and they’ll tell you when to open the window. The ergonomics of how you’ll use it are maybe more important than accuracy when it’s <200ppm off.