By saying that it cost around 5€ I meant that the material to build this cost me around 5€. I would happily set them up for free if one gave me the material for free, but the microcontroller and so on do cost some money.
The API and (of course) the source code are free (as in freedom, being AGPL licensed). And since I did my best to document it well and provide easy to follow instructions in the README, I do think I managed to make it free as in freedom.