Did something similar recently for fun and cost was ~20e for co2 sensor, attached to a raspberry pi pico or esp32 and it ends up being 27-30e. (add some more sensors like co or gas for 10e each, a mini display for 5e, hook up to home assistant or telegram and ends up being more useful)

To be fair, QA on these things tends to be valuable, so perhaps cost might be justified.

Cost likely can go down if you know what to buy, I didn't and just bought what a reputable seller had.