When I considered building a meter, the sensors were about £10 to buy just one at retail price.
You see, el cheapo NDIR sensors like the one you just referred to are basically copies. Aliexperess is flooded by them. They are slow, inaccurate, unreliable. They usually have a detection range of 400-10000ppm or 0-5000ppm which technically is fine BUT with a very poor accuracy which can range from 100ppm to a 1000ppm 😬. There are not many companies producing quality NDIR. I suggest looking for Sensirion, Winsen, Cubic, Senseair. All produce quality, reliable sensors. Some are very expensive some very reasonable, but nothing like £10, that just wont happen and I wouldn't waste my money on those copies.
Even the kit you linked to has the option of with or without the sensor, and the price difference is only $20.
The component alone will cost ~30/40USD sometimes more depending the manufacturer.
The Kit I posted earlier is the cheapest you will find with a good quality sensor. They use the Senseair S8 sensor which retails for 25/30USD depending where you buy it from. AirGradient is able to get it for slightly lower because they buy it by the hundreds.