Even if the cost of the cars you choose only decreases by a few thousand dollars because you choose to bike, it can significantly tip the scale for cost savings, with a significant bend point when the value of the car is low enough that you drop full coverage and another significant bend point where you own fewer cars. Cost of parking at work also can have a significant impact on the available savings to a bike commuter. I'm with ender on the exercise gained negating any extra time taken making all cost savings a plus with no time downside.
And that's all fine and true, but for someone who doesn't bike commute, telling them bike commuting a few days is a huge financial benefit because maybe they can go down a car is a stretch. You're assuming a huge level of commitment from someone currently not biking at all in order to really see a return. I could bike commute a few days a week a few months a year. But all that really does is save me a couple bucks a day in gas and pushing out oil changes and tires a few weeks. I cannot give up a car, because my spouse and I work in opposite directions and we share responsibility for collecting our daughter from school, which can't realistically be done via bike given the hours the school is open (I work 8:30-5:15-530 most days, and school opens at 8:10 and closes promptly at 6, it's just not reasonable to build in additional time for biking and maintain my work hours and school hours).
Basically not bike commuting is a commitment of 0/10, commuting a few days a week in nice weather is a 2/10, and giving up one of 2 cars is an 8 or 9/10. Most people are not going to get from 0 to 8-9, and that's where you actually get material savings.
I don't really see how you only get material savings at an 8 or 9 out of 10? I have a pretty small car with good gas mileage. If you estimate $0.50 per mile, the round trip 20 miles is $10 a day. That's not nothing.
It comes out to maybe $8 a day. I figure gas is part of it, and wear and tear. I don't factor in insurance because I'm paying for it anyway. Then, if you have a car that you paid $20k for, and you keep it for 200,000 miles, that's 0.10 a mile. The less you drive it, the longer it lasts and the longer you can go before replacing it.
Now husband and I work near each other, so we used to each bike one way. We biked a little bit after kid #2 was born. But then it got to be logistically difficult. Even though we have flexible enough schedules -
-Kid #1 drop-off is at 8:20, pick up is before 5:30
-Kid #2 drop-off is at 8:30 approx, pick up is before 5:30
-Kid #2 daycare is 5 miles in the opposite direction from work.
We could do it, but it lengthens the day (basically, whichever person goes in "early" bikes in and the other does dropoff. The one that went in early does pickup and the other could bike home).
However, to work a full day, that means the person biking home would have to work until at least 6 pm, and wouldn't get home until 7 pm. That's sort of late for dinner, and add in baseball practice and music practice to the mix...It was much easier for us to manage it when I was working part time. It's kind of like playing calendar tetris. And I just don't have the energy for it right now.
This fall kid #2 is moving to a preschool that is between home and work. The following year, we will have one GLORIOUS year of both kids in the same school.
I imagine that we'll revisit biking at that point.