K30 is not an inexpensive sensor, that's true.
I actually tried a few low cost analog electrochemical sensors, but I found the whole calibration process suboptimal - to say the least. After doing some reading, I decided to try a NDIR sensor. I really liked the K30 because it has digital output and it is easy to use (RS232). It is also an auto-calibrating sensor.
K30 has some good characteristics. (measurement range: 0 – 10,000 ppm, accuracy: ± 30 ppm ± 3 % of measured value within range).
It is the same sensor one can find in a lot of devices like this one:
http://www.amazon.com/Indoor-Air-Quality-Meter-Temperature/dp/B001PDGFR8 (I know that because I opened a few of these guys).
I think I am providing good value by pricing the K30 sensor at much lower cost then the retail value of this sensor which is around USD 80-90.
I also think LiV Pi Expert provides good value compared to the IAQ monitor I mentioned before: that one is a regular, stand alone device. I have seen similar devices with additional data logging capabilities (this means connecting a USB cable from the device to your PC to download and visualise historical data) that sell in the USD 175-250 range. LiV Pi is an Internet connected device (website, APIs, Instant Messaging reporting, etc..) with capabilities that far exceed those of all stand alone IAQ monitors.