Let's see....used bike from goodwill $16
2 new chains (1 when I first bought the bike and the second after 2000+ miles on the other chain) $40
1 set of tires: $40

Total: $96

Car gets 30 mpg so 4000 = 133 gallons. At 2.00/ gallon = $266
One oil change = $30
So far, $296 for the car and I haven't taken into account wear and tear. So, yes, commuting on a bike is a lot cheaper.