There are a few factors. The gas sensors you can afford as a (poor) hobbyist tend to be resistive/heater devices. You heat up a catalyst and monitor its resist

There are a few factors. The gas sensors you can afford as a (poor) hobbyist tend to be resistive/heater devices. You heat up a catalyst and monitor its resistance which varies when a suitable gas is present. The downside is that typically they're sensitive to all kinds of gases - e.g. methane, ethanol other volatile organic compounds and so on. Either you can use them in a single-gas environment, or where you would only expect one polluting gas, or you can have a general threshold. Cheap and cheerful and nice for IoT home "air quality" sensors, but useless for real measurements. Most of the very expensive sensors - like the one you linked - have much higher specificity. That methane detector is optical and is tuned precisely for the IR absorption band of the gas. Optics and filters are expensive, plus the driving circuitry and probably NIST-traceable calibration if you want it. EDIT: It also uses a fairly unusual LED wavelength which adds to the price (InGaS = money). Other sensors, like Oxygen, have shelf lives of a few years and need to be replaced even though they're horribly expensive ($200 or so). A filtered methane sensor is about $50 (Figaro make a lot of nice gas sensors): http://microcontrollershop.com/product_info.php?cPath=301_428_579&products_id=6786 http://microcontrollershop.com/product_info.php?cPath=301_42... AMS also make some nice inexpensive general VOC sensors like this one: http://www.digikey.co.uk/product-detail/en/ams/AS-MLV-P2/AS-MLV-P2CT-ND/5117220 http://www.digikey.co.uk/product-detail/en/ams/AS-MLV-P2/AS-...