With CO2 sensors unsurprisingly you get what you pay for. Stuff that is accurate to ±6ppm will set you back about £3,000. Sensors that are ±70ppm are about £70 or less (they often appear on a well known auction site). Both these use a principle called Non Dispersive Infra Red (NDIR) and are what is generally used in commercial and agri-business applications. If you use lower cost parts based on other technologies they will be susceptible to cross sensitivity from other gases (including water / humidity), more suceptible to signal drift, less acurate as calibration is expensive etc.
Our T6713 or T6613 parts would work well in your application, and there is some code on github already available (search AmphenolAdvancedSensors/Telaire).
Good luck with your project.