People who say that "bike commuting doesn't cost less because I buy all this stuff" are forgetting that, for the most part, most of that "stuff" is personal preference and not necessarily needed for commuting specifically. In the winter when I only rarely commute by bike, gasoline costs run about $50 a week. That's not just for the commute, admittedly, my husband drives the van everywhere. If I did have a separate vehicle to drive in with, I'd pay $40 a month for parking in addition to the gas, insurance, and maintenance costs.
I can't even remember what I paid for a lot of my commute stuff, I bought it so long ago. Panniers - at least 22 years old.