Low power, cost effective way to measure CO2 (or equivalent CO2)?
£5.96 ($7.74 USD) is actually incredibly cheap for a C02 sensor. I used a Futurelec C02 sensor in a project, and it runs $34.90 with no processing capabilities at all - just a simple voltage output. 

You'll have to read the datasheet for your specific device to see what "equivalent C02" means. 

Here is a PDF from Gas Sensing Solutions, a UK-based sensor manufacturer, and they say:


  Some manufacturers of air
  quality sensors are now providing an
  output in "CO2 equivalent units." This
  measure is considered misleading and
  may confuse many new to the indoor air
  quality industry.
  
  There is currently no recognized procedure
  to directly quantify the output of
  these sensors to carbon dioxide or the
  differential between inside and outside
  concentrations that is used to determine
  ventilation rate.


...so read the manufacturer's documentation carefully to fully understand what you're actually measuring.

I'm going to deviate from objectivity for a moment and give you an opinion: I'm skeptical of the devices you linked to. I have trouble believing they can deliver a high level of accuracy for that price, given that bare sensors alone (as you noted) are significantly more expensive. The fact that they provide a dubious "C02 equivalent" measurement rather than an absolute value makes me question their usefulness. I would want to learn a lot more about how they work and exactly what information they're providing before I purchased one. I'm sure someone with more experience working with air quality measurements could provide good use cases for these, but I can't help but be wary of them.