You're getting downvoted, but for real this is so disconnected from what I'd say is feasible for most people. I and most of the people I know live squarely paycheck to paycheck, I can't afford to buy my friends birthday presents at all, let alone set aside that much per month just to put batteries in other peoples' houses. This feels like one of those things where the author doesn't grasp the privilege he lives with.
Even where people probably could afford it I think spending €100 on a birthday gift would be excessive. It would be showing off.
Surely not for _most_ people, but for people on Hacker News, in the correct age range to have a house, putting 100e aside really does not seem out of whack.
Maybe the author is privileged and it's fine, it's not because you personally can't afford any gift that the author is disconnected, he might not just be talking to everyone on earth.