Although a bit above $200, but I own an Aranet4 CO2 monitor (it also measures humidity, air pressure, and temperature). It has a great app that allows you to se

Although a bit above $200, but I own an Aranet4 CO2 monitor (it also measures humidity, air pressure, and temperature). It has a great app that allows you to see historical data too. I've been using it for a while already and I'm really happy with it.