The way I'm looking at it, people choose to bike commute for one of these two reasons:

1) It saves money for their situation and point of view
2) It just feels good, with or without any scrutiny of the costs

For me it's the second one. If I factored in the amount of overtime pay I'm sacrificing by bike commuting (once or twice a week, 20 miles each way) then there's no way I could conclude that I'm saving money. But OTOH, doing this is a great way to get in some riding during the week which I otherwise wouldn't be able to do.