CO detectors (Carbon Monoxide) and CO2 (Carbon Dioxide) are completely different. You can buy inexpensive $45 dollar CO detectors at a hardware store but they do not tie into a system they just sound locally. A system detector will cost around $100-$150 and a carbon Dioxide CO2 detector/sensor can run over $500-$700 each.
A Methane & Propaine detector can run over $120-$180
A Nitrogen Dioxide detector can run over $700-$800
A Combustable gas detector can run over $500-$700
A Hydragen Gas Detector can run $400-$600
An Ammonia NH3 Detector can run over $700
An NO2 Nitrogen Dioxide detector can run over $700-$800