I am cheap about my commuter car. I am cheap about my commuter bike. Here is what I figure for each of these forms of commuter transportation.
Car
$206 insurance for the year (probably a bit higher since I get a multi-car discount)
$85 license fees
$45 (2 oil changes)
$600 Other expenses like car repairs etc. I figure if I can drive a car for $50 a month in this department I am doing pretty good.
$906 total
Additional
I'll probably drive about 2500 miles this year. Work will probably give me $150 for mileage maybe more so that lowers my costs I guess, but I can't transport other people on my bike unfortunately. =)
I paid $700 for the car about 3 months ago and have $850 into it including some work and the sales tax I had to pay when I purchased it.
Bike
I already have it, but I paid $20 for it like 2 years ago.
Everything else I put on it has cost at most $40 in the pat 2 years.
So basically however you want to cut it we are talking very little money this year on the bike.
I will probably end up riding somewhere around 800-900 miles this year maybe more depending on the weather. Next year will be significantly more.
Other than the bike and any other maintenance there isn't many other costs. Exercise saves me time and money no extra charge for food. I don't wear much cycle specific clothing and if I do its stuff I have had for a while. I guess I did buy a used raincoat and pants set for $7 at the thrift store so I guess I am over $60 spent. LOL Even my helmet doesn't cost me anything because my health insurance pays for it.
Bottom line or me there is no contest the bike wins hands down. The car is needed and handy for some things though and ultimately isn't that costly so hopefully I don't have a major failure on it any time soon.
A couple other comments. I have spend significantly more time working on my cars than my commuter bike. I have been fortunate to do all my repairs on both the last 3 years. The bike is pretty simple. Cars are too for the most part, but it takes alot more time and costs alot more when something cost wrong. I work on bikes as a hobby so I have plenty of spare parts lying around that are basically free so that helps alot. I'll also say I am fortunate that I can get by with crappy junky cheap tires and I don't have flats. Obviously thats not the case for everyone. If I had to purchase some high quality tires my expenses would almost double. My commute is only about 4 miles, plus whatever errands I may run on any given day so in general I am not putting a ton of miles on either my bike or car. I do use other bikes for my commute, but 90% of my mile is on the old Trek.