I'm with the "bikes cost at least as much" crowd here. I only have a 3-mile round trip commute, so gas cost would be negligible. The car I would use if I still had it would use about a gallon of gas a week for the commute and was completely paid for (the car was a freebie hand-me-down). Parking is $30 per month, so I'm really saving <$100/month counting insurance cost. So that's conservatively $1200 per year, but I spend at least that much on my bikes. But since I don't have my own car, that's just a displaced cost rather than an additional one. If I didn't tinker so much with my bikes the costs would be significantly lower.

I've actually been trying to financially justify my case for going carfree (15 months now) lately, and I really can't. An old beater car doesn't cost too much to run since I'm a capable mechanic. The distances I travel are generally less than 5 miles at a stretch, so gas, tires and other consumables would be minimal. I spend more time in transporting myself by bike and mass transit than I would if I drove (the commute excluded, what with engine warm-up time and the fact that I'd have to park a block away). The only reason I've been able to come up with is that carfree is just a whole lot more fun! Every day in the city without the insulating convenience of a car can be, and usually is, another day of adventure and interest. And with that $1200 I can build myself and my wife at least one upgraded bike every year just to keep things from getting boring. Try doing that with cars and the costs get out of hand really quickly. Like Alexey says, the best things in life are free (or at least really cheap).

I'd like to add that I'm carfree more as a result of circumstance rather than free choice: the damn thing broke down at a financially strapped point in my life, and since I was 75% car"lite" anyway I just decided to let it go rather than hassle with replacing an oil pump. I'd much rather wrench on bikes than cars any day.