If I had a car rather than a bike, then the expenses that I now have that I would not have otherwise are:
Bicycle maintenance costs: Estimate $200/yr
Local car rental (Avis and Zipcar): Estimate $200/yr
Public Transit costs: Estimate $100/yr
Gym membership where I shower in the morning after riding in to work: $320/yr
Not purchasing things in bulk when they are on sale: 50/yr
So all told, probably on the order of $1000 per year is spent on things that I would not need if I had a car.
If I had a car, my estimated costs would be:
Fuel: $2400/yr (based on 20mpg, 12,000 miles per year, $4/gallon)
Maintenence: $500/yr (wild guess)
Insurance: $1200/yr (wild guess based on what I hear colleagues complaining about)
Depreciation: $1500/yr (based on buying a $30K car every 10 years, and selling it for $15K at the end of the 10 years)
Registration: $250/yr
Total about $6000 per year
So I figure that in direct expense, not having a car is saving me about $5K per year. To that I would add the better health that I enjoy, and I am waaay ahead.