I have a dedicated commuter bike, which has about 40,000 miles on it since I bought it 8 years ago. I've bought a couple of wheels for it, some accessories, tires, tubes, chains, cassettes and do all my own maintenance. The bike originally cost me about $350 new. I've spent maybe $800-900 on it over the 8 years.

about .03¢/mile

My commute costs are low per mile because I buy quality equipment at as low an initial price as possible, maintain it myself and ride it a minimum of 100 miles/week consistently.

Given all that, commuting by bike is a priceless experience and not worth counting pennies about in an obsessive way unless it makes you feel even better about riding your bike.