I have kept meticulous records of all of the expenses I have had over the past 8 years. I know, it verges on obsessive compulsive behavior, but it is also interesing.
I have commuted full time since July 1997.
Total spent (including $800 Trek 1220): $3688
Total # of days commuted to work: 1408
Total miles ridden 35,382
Cost per day commuting $2.60
Cost per mile $0.10
By way of comparison, public transportation would cost me $8.80 per day for a totoal of over $12000, and if I bought a car and parked, etc. it would have been over $65,000 over 8 years. Not to mention that it is faster and more fun.