In terms of environmental cost (not just CO2) a bicycle is actually 3 times as expensive as a really good car: http://www.thetruthaboutcars.com/2011/09/is-a-bicycle-really...

And if you put 4 people in that car, it wins easily, even with a somewhat more average car.

People like bikes mainly because people eat and exercise anyway, so they might as well do some useful work for it. But purely from the environment, and assuming people only bicycle for need, not exercise, it's not an automatic win.