Bicycle commuting cheap, but not without its problems
Perhaps $4.00 gas has caused you to consider bicycle commuting. Good for you! A nice spin on a sunny summer morning is a marvelous way to start the day, and the return trip is a great way to burn away those work frustrations.
However, there are some expenses you may not anticipate and hurdles you'll need to overcome, ones that mitigate some of the benefits of bicycle commuting, ones that the bicycle industry won't tell you about.
Bike run cheap, but not free. A $20 tire subjected to city street debris may last 1-2,000 miles, an inner tube half of that before it is too patched to depend upon. A good raincoat is an essential, as are gloves and some rain cover for your shoes. Add a helmet, a good lock, bags to carry your clothes and incidentals, a repair kit adequate for typical bike problems (tire patching, drive train adjustments, wheel truing), lights and reflectors, fenders, and you can run up a tab of several hundred dollars.
Also consider the money cost of time. Sure, you're getting exercise as you ride, but you'll spend three times or more as long in your commute, time that will have to be shaved off your normal routine. Add to this time spent fixing the occasional flat.
You'll need a place to safely store your bike at work and a locker room where you can clean up (yes, you'll be whiffy after the ride, and you should do what you can to freshen up before subjecting your office mates to your funk). Forget about driving very far for lunch; the hassle isn't worth it. Ditto stopping to shop on the way home for more than what you can fit in your panniers.
If it's raining, you'll get wet. Wet clothes need to dry, so you'll need a place to hang them at work. Riding in the rain isn't much fun, especially in traffic. In heavy traffic, expect cars to cut you off or gesture angrily when you hold them up for a nanosecond.
If, after considering these factors, the idea still appeals to you, then ride on. You have the spirit to make it work for you, and will earn the right to laugh as you ride past gas stations.