The reimbursement rate for driving right now is $.51 / mile (http://www.irs.gov/newsroom/article/...240903,00.html). So, your commute probably costs you about $11.22 per day all things considered (the reimbursement rate takes in to account things like wear and tear, etc.). I don't see how you could possibly spend that much on your bike commute in each day. Once you have your biking gear things will become pretty static in terms of cost. I get 2,000-3,000 miles out of a set of tires on average so they don't cost too much, I sometimes bust lights and have to replace them, etc. but that's about it.

Also, do you have to pay to park at your work or is parking free? For me, parking is like $11 a day that I don't have to pay.

One other thing you could factor in to cost would be long-term decreased healthcare costs. I'm not sure how to quantify this one though.