I have a 2 mile daily commute M-F (plus an average of 2 additional miles a day) and a total 20 mile commute on Sunday (when I go to church in a neighboring town). Here's how it breaks down for me:

Car payment for a beater from a family friend = $100/month
Gas for beater = $30/month
Insurance = $200/month(!) (I'm a nearly 21 year old male)
Periodic Maintance = $14/month (oil change every 3 months and monthly car wash)
Unplanned Maintance = Who knows, last time it was a new radiator for $200
Parking at university = $13.33/month ($120/academic year)
Car Registration: $35/year

Total monthly = $360.25
Total yearly = $4322.96 (excluding unplanned maintance)

(Excluded: Driver's licence, $35 for 5 years. I'll keep that even after I make the switch to biking exclusively because I may need to borrow a car or something in the future.)

My new bike, when I get it in the next few weeks as quoted my my LBS:

Specialized HardRock Comp Disk: $600
Lock: $70
Helmet: $30
Multitool: $30
Slick tires: $40

I'm not adding maintance or tune-up costs because I'm not sure what those will be yet, but I know they'll be cheaper than the beater-car. I'm also not adding in the cost of better, more nutritious food I'll end up eating to keep my performance up (which should be minimal). It's easy to see that I will still spend much less biking than driving.

As for health benefits, my mom had her first heart attack at 44 and died from coronary problems at 53. She didn't exercise or eat right. Looking at it from a purely economical standpoint, I'm sure her many surgeries, medical equipment costs, and hospital stays cost well over $100,000. Looking at it from a purely emotional standpoint, I can not put my future family through what I had to go through and will spend whatever is needed to keep that from happening. That negates all of the above in my eyes.

Also, the OP has a flaw in his/her logic: $2 each way for the bus is expensive, but that's the non-regular user rate. I'm sure a bus pass would be less expensive, especially if paid for many months in advance.