I pay less than 90 euros/month to get most places in Paris and its suburbs (and that's about to go down to 77 euros/month with access to a much wider area). I can't even get a parking space that cheap. Add in the cost of insurance, gas, the car itself and it's a no-brainer. In the rare case that getting somewhere 10 minutes faster is important to me, or I'm going home after most transit has stopped and I won't spare the time for the night bus, I've saved plenty to spring for a taxi.
But it's not just that it's an urban environment. I used to live in a small town in Belgium and I managed to get almost everywhere I needed to in the country -- not the town, but the entire country -- on public transit, and often faster than if I'd had a car (i.e., pretty much anytime I had to travel through Brussels during rush hour or an event that attracted international dignitaries).
Contrast that with Atlanta, where I owned a car primarily so I could travel less than 4 miles to work along a route that was served by a bus that often, but not always, showed up. Even when it showed up, it only showed up once an hour. And if I wanted to use MARTA (Atlanta's version of a subway), I needed that bus, since MARTA hardly goes anywhere. Granted, Belgium is a small country, but it still seems wrong that I could travel halfway across it -- the whole country -- in the time it took me to get from Virginia-Highland to Midtown on public transit.