I have been commuting for free for a couple years now. I ride cheap bikes ($100 or less) for the most part although the mini velo I bought a couple years ago was $240 but its not a primary commuter anymore. Its the non-commuter bikes that aren't free for me, but my bike flipping habit has paid for them and more over the years. There are really only 2 bikes I own that I don't throw into the commuting rotation now and then.
My commute isn't very long and it got shorter just over 3 miles with the new job so I don't rack up the savings very fast even if I rode everyday. On the other hand I ride cheap bikes that I work on myself have parts laying around to maintain them and buy when stuff is on sale and don't have need for expensive tires like some of the others do. I enjoy the bikes I have although the beater I was using I realize needs to be upgraded its a decent bike and I like how it turned out the way I built it up, but I need more than 10 speeds. For now I am back to riding my old Trek Police bike as a winter bike. I forgot how great it rides and I have probably put less than $100 into cost of ownership since I bought it over 3 years ago and I've rolled up plenty of miles on it. Plenty of people can get by on much less than some of the budgets outlined here, but there isn't anything wrong with spending more and enjoying it.