I find that this year is working pretty well, since last year was when I bought the bike, clothes, lights, and all that. This season (I kinda look at commute season being started in the fall, when I don't get much recreational riding in), I have had to buy a tire. So I kinda figure that I save straight up a gallon of gas and a little bit each ride - about $5 a ride is my estimate. It's probably a bit more, since I drive less still, since I don't make side trips on the way home and am loathe to get back in the car once I get home.