[svl]

No one uses helmets. Most people don't use panniers either; small loads are attached to the back of the bike by the default "snelbinders" (elastic straps). Or what's really popular lately are plastic crates above the front-wheel. Locks and lights (and reflective strips on the wheels) are standard on all bikes. In cities you'd get a second lock, but that's really about it.

[Freak_NL]

As for the foul weather clothing; you would need a raincoat anyhow and a pair of rain pants won't set you back that much.

I spend about €15 a month on my bicycle if I spread out the cost of purchase, clothing, and maintenance (mostly parts, I do my own maintenance, which isn't uncommon here). That's for a high quality, extra tall bicycle (I'm 2m tall) that looks presentable enough for my daily commute to work.

A student could probably manage with €3 to €8 a month.