Incidentally, humans can tolerate much higher CO2 levels with accommodation, but, as with altitude, it takes days. The US Navy submarine people have done considerable work on this. So this is not a global warming issue.
Real CO2 sensors are still about US$60 as a part. The standard sensor has a heating element, and uses about 75mA, so this isn't suitable for battery operation. 1mA sensors are now available, but for US$160 or so. This cost problem is holding back widespread use in HVAC systems. HVAC systems for rooms where the people load varies widely (classrooms, offices, hotel function rooms, restaurants) should have a CO2 sensor to tell how much outside air you should be pulling in. But outside of hotels, they're rare.
Cheap "air quality sensors" measure volatile organics, which is something else entirely.