Comment by aurelwu

you can get a ESP32 and a CO2-Sensor, easiest would be to use a development kit like a m5-stickC and the Sensirion SCD41 and with some small arduino scripts you are good to go to send the data via WLAN or BLE for something like 70€. Can be even cheaper if you don't use a development kit but it makes things a lot easier and the small display helps with debugging.