@gsuberland @cos @joepie91 ahh, that's a bit too overboard price-wise unfortunately :(
@Violet For Operame (operame.nl) we used a MH-Z19B (and later, an apparent clone) which is also an NDIR sensor AFAIK. Not the highest-quality or most accurate one but it was definitely good enough for the purpose. The whole Operame unit (sensor + display + microcontroller + PCB) ended up costing around 30 EUR in parts I believe.
@Violet (Commercial products using that sensor will probably be more expensive though, because of markup...)