Today? Wow. You must have very weirdly expensive batteries and/or electricity. Around here, a complete replacement of non-rechargeable bike light batteries would cost €1.25, and they'd still last more than one ride each. Rechargeable batteries may cost more, but last thousands.

That was quite a while ago. I can't recall exactly anymore, but the yearly battery cost was probably indeed quite low, ~20 euro maybe? They just didn't fine people often enough.