Nice build, simple board too, the author basically makes a shield, or hat for Raspberry Pi to help interface your heating/cooling systems. If you already have a RPi, you can by the shield and materials four I think $50 US? You can buy the whole thing ready to go in a case for $100 EURO. Which I think is around $150 US, cheaper than a NEST.
http://www.instructables.com/id/HestiaPi-Open-Smart-Thermostat/