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It's a bit tricky, because one of the most expensive parts of an air quality meter appears to be the CO2 sensor. Less expensive sensors approximate CO2 concentration (it's referred to as "eCO2") and can be wildly off due to the way they sense gas concentrations.

If you have the equipment to make electronics at home and are willing to DIY, it can be done for less than $100 and will be comparable to an off the shelf unit.

I’m partial to the no-name monitor described here since all the sensors are removable and can be moved into a DIY unit if it breaks. The sensors used are the same parts used in big brand monitors.

It also helps that the processor is unlocked and custom firmware can be cooked up:

https://frdmtoplay.com/patching-in-fahrenheit/

Those are subjective but there's a good paper on accuracy here https://www.sciencedirect.com/science/article/pii/S036013232...!

tldr for CO2 best is https://www.kaiterra.com/en/laser-egg-co2