Ah, a russurected thread. Well I wasn't around when it was born so I'll post on it.
Before I started bike commuting, I was using public transportation, which my job paid for then and would still pay for if I used it as my main commuting method. They will not pay for my biking. (I should admit that they do provide free bike racks, but I use a locker that I rent for 50/yr, and that there is a fitness center I use for changes and showers at ~25 a month, I doubt that it is a money making deal for them). So commuting to work by bike really isn't an economical decision for me unless perhaps you start speculating about health benefits and future health care costs. I first did it in order to get a little exercise and to have a little more autonomy that I have with public transportation. Then I found that it was just fun and I became aware of the environmental benefits. Since I started commuting, I got rid of my car and some utility trips are done on the bike. So overall I think there is an economic benefit. But speaking strictly of commuting motivations, in rank order,
exercise,
fun,
autonomy,
environmental.