Hey,
First, in the name of full-disclosure, I do work for CO2Meter.com, so take everything I say with a grain of salt 
, but I'm also a tinkerer myself and do love the Arduino platform.
I see what you're trying to do. CO2 is most definitely a fresh air quality indicator. The premise is that in a closed room the CO2 will rise (we exhale it) and it indicates that the room has stale air in it. Commercial forced-air ventilation systems integrate our sensors regularly for on-demand ventilation to save energy by only pumping in fresh-air when the air is saturated with CO2.
We offer some of the most cost effective sensors on the market, but unfortunately we can't touch $35 a unit. The absolute cheapest sensor would be the K22 model here:
It has a PWM output, but I'd recommend using the I2C interface. You'll have some issues with doing 10 meters of I2C communication, The inductance of the wire might blur the bits, so to speak, so you might have to get creative with a circuit to boost the voltage / use a differential signaling method such as RS-485. I'd recommend the K30 sensor, which features a UART engine that easily converts to RS-485 with a transceiver chip, but it won't meet your budget.
Let me know if you have any questions!
Andrew Robinson
Integration Specialist