For $20, you can get the MH Z-19, which is a NDIR type. You can talk to it over UART, and it comes in a 5000ppm and 2000ppm model IIRC. Decent documentation, and a good little sensor.
The S8-0053 looks interesting, but I have no experience with it. Also NDIR, boasts +- 40ppm accuraxy -- the room air might not even be that homogeneous...
The cheap ones are electrochemical, and they're not even that cheap.
Problem with it is that it autorecalibrates itself every 24hrs as far as I remember. It is a problem in room monitoring where you have constant high CO2- you'd have to take the sensor outside to fix it's reference level.
The S8-0053 looks interesting, but I have no experience with it. Also NDIR, boasts +- 40ppm accuraxy -- the room air might not even be that homogeneous...
The cheap ones are electrochemical, and they're not even that cheap.
https://sandboxelectronics.com/files/SEN-000007/MG811.pdf