Cheap PPM meter?

Why are those things so expensive?! I'd like to know what kind of PPM I'm running, but no way in hell can I afford a $300 piece of equipment on a whim. Anyone know of anything cheaper/simpler? I Just run it with this co2 calculator I found and I dont have any complaints, I would just kind of like to know exactly where the PPM's are at.

In the $150 range I've seen thermometers that have a function to also measure co2 levels. Can't comment on accuracy or quality though.
http://www.amazon.com/Indoor-Air-Quality-Meter-Temperature/dp/B001PDGFR8