The whole idea of this project was to make it cheap, so that most people would afford it. I have now purchased all the parts for the second prototype, and have an initial figure:

$ 54.7 or € 48.8

See the BOM for the complete list and calculations.

Some notes:

• The prices are excluding taxes.
• All prices are taken from Digikey, a few days before posting this.
• This is a prototype, and there is room for optimization in terms of price. As an example, the push button currently costs over $2 because I chose a surface mounted version. I can get it for one tenth of that if I allow through hole.
• The solar panel is probably over-sized in comparison to the power consumption I have in mind. It is about $12, and I think a much cheaper one can be used. Time will tell!
• I'm so far assuming that you or a friend has a 3D printer (latter in my case), and I have not included the price of material and electricity for printing.
• I'm also assuming that you have a computer fan laying around, to be used for the anemometer. Quite a stretch, I know. I'm still not convinced that getting energy from the wind will work out. If it doesn't, I might combine the wind vane PCB with anemometer, which would save a lot of money (not only the fan, but also circuitry on the main PCB). In any case you can buy a small fan for € 6 at digikey, if really needed.

If I would have stated € instead of $ in the begging it would look better, but all in all, I'm very happy with this initial figure. It proves that my initial statement was not too far off.