I've got an old beater car, but even before I started commuting, I wasn't driving it to work. Traffic and parking hassles in downtown DC made it not worth my while. So, I was taking the metro (and sometimes additionally buses) to bet from home to work and back. All that added up, on average, to around 6 dollars a day. Figure 20 working days a month, that's $120 a month in savings. Metro fare came out as a pre-tax benefit, so it's probably actually closer to $80-100 a month out of pocket for me, but still a decent savings.

Alas, if I didn't have other pressing financial needs, I'd happily be reinvesting much of those savings into some stuff for my bike, but even assuming I were to buy all the accessories and stuff I want, I think they'd be paid for in just a few months of not paying that Metro fare.

Apart from accessories, I've got some wheel repair/replacement issues to deal with right now, so that'll end up running me anywhere from $30 to $120 depending on what I end up doing.

Other than that, I don't foresee any major expenses. So let's see.

$225 = cost of bike
$200 = wanted additional accessories, upgrades, tools, etc.
$100 = wheel repair (near worst case scenario)
$525

That's about five months of commuting and I'm paid for. Then I'll start to save money. And that's not including the health benefits, the pleasure I derive from bike commuting... Not too shabby.