> As I'm surprised your socks/underwear don't cost at least €35 per year, eventually they get holes in them.
I figure holes in socks are heavily dependent on slipper use. I wear slippers 95% of the time indoors, and my regular-rotation socks that I wear every couple weeks seem to be lasting 15-20 years (so, say, 200-250 days/washes since I wear different socks seasonally) before their elastics fail.
The average is a bit of a weird number... e.g. I buy a couple weeks' worth of underwear at a time, and they last a decade or so. So 9 out of 10 years my cost is $0, but my yearly cost is in the $30 range.
edit: And as noted by another comenter, I hang dry my socks/underwear, which presumably contributes to longevity.