It depends strongly on the gas mileage for the car, of course, but my first assumption would be that for most people the cost of public transit is about the same as gas. My daily commute, for example (which I know you don't have, but most people do) is about 50 miles. On days I don't work from home, my choice is between a commuter train which costs $22 round trip, commuting on the long-distance train ($28 round trip), or driving, which costs about $10-15 in gas, depending on the current price of gas. I mostly take the train, because driving into Washington, DC isn't fun; I'm certainly not trying to convince you to run out and buy a car, just responding to the assertion that it would cost $7000 a year extra to keep one.

I lived in the DC metro for two years without a car, before purchasing this one, and having lived both ways, I personally strongly prefer having a car. Without one, I had to structure my life around not having a car. Going to the computer store (Microcenter) to pick up a new video card on a whim was something that would take the bulk of the day without a car (two or three metro lines depending on route). If the store half a mile from my house didn't have something, I had to either plan a trip on a bus to another one (and what if that one didn't, either? So I spent a lot of time trying to figure out in advance online if they did), or just go without. I don't much like shopping for groceries, but I got to do it three or four times a week anyway, because I couldn't carry enough 2L bottles to last longer than a few days, etc... without a car, a lot of my life was concerned with inventory and transportation, in a way that I don't even think about with a car, since I can pop out to any kind of store and be back in 20 minutes.