you assume that the error will always be in one direction

and if sometimes you ventilate a bit sooner than required, at 700, what?

businesses will not put $200 meters in every room

Have you looked at the prices of meeting room furniture? A $200 meter is not a significant cost measured against what it costs to furnish the room in the first place. It only becomes significant is you treat it as a line item disconnected from the room it's in

businesses will not put $200 meters in every room

There are good $50 Euro meters. Besides that, I am not sure if that is true, at my wife's workplace, they put high-end CO2 meters in every larger room where multiple people meet. Admittedly, this was during COVID, so a lot of organizations were using CO2 levels as a proxy for finding whether a room was properly ventilated.

$200 is nothing compared to the lost productivity.

Presumably there is still the need to ventilate. So the concentration can also be measured more centrally. That is how the mechanical ventilation unit in my house works. For both humidity and CO2.

You put one CO2 sensor in the return air duct and tie it to outside air control.