And how much power does that box consume ? The average price of power during a year here is around €0.45/year, so a box drawing 50W (not unlikely with a decade

And how much power does that box consume ? The average price of power during a year here is around €0.45/year, so a box drawing 50W (not unlikely with a decade old processor and two disks) will use 438 kWh during a year, meaning it would cost €197/year to keep it powered. Add to that the ~€150 x 2 for new drives, and you end up with €227/year, or €19/month. But of course, if you, as you stated, had the box running anyway, the math works out differently as you're essentially splitting the cost with whatever purpose the box already fulfills.