Ken Kifer wrote a very good article about the cost savings of bicycle commuting vs. automobile use/ownership. He calculated in great detail a savings of about $7,000 per year in 1997. This is a well thought-out and researched article with a lot of support info.

See http://www.kenkifer.com/bikepages/commute/quadrupl.htm

Dave Glowacz of the Bicycle Federation of Wisconsin wrote a similar article in which he estimated that the cost savings was around $3,300 to $6,500 per year.

See http://www.bfw.org/bfw/savetime.htm