I can share a study that some real economists published!
In the most recent Environmental Health Perspectives, some folks from University of Wisconsin-Madison, University of Iowa, and Georgia Institute of Technology estimated that $8 billion/year could be saved by making 50% of short trips by bicycle. In their analysis, they evaluated 11 of the largest mid-western urban areas. These cost savings would be seen through improved air quality and physical fitness (i.e., reduced health care costs).
Since most car commutes are just shorts trips, riding a bike for half of those trips could have a big impact.
(In case you needed some extra motivation to jump on a bike this winter!)
Grabow M.L., S.N. Spak, T. Halloway, B. Stone, A.C. Mednick, and J.A. Patz. 2011. Air quality and exercise-related health benefits from reduced car travel in the Midwestern United States. Environmental Health Perspectives 120:68-76.