I live in San Francisco and walk/train/bus everyplace. (I do not have a bike) When needed I have access to car via a car sharing program. For 50$ a month I get a pass that gets me anyplace (within the city) with in a relatively timely manner. I have access to a car sharing program that regularly costs me 50$ a month or so on average. Throw in a rental car every two months for a weekend at 100$.
So it averages out to 150$ a month (gas included) to get me every place I want to go.
But really what gets me is the lifestyle benefits, I never have to worry about parking/oil changes/gas prices/insurance nor drinking and driving. I walk a lot and it keeps me looking good and in great shape. Not to mention walking is very relaxing vs driving, I read and listen to pod casts. So not only do I save a lot of money (vs a 500$ monthly car payment), I've greatly reduced my carbon impact, I have less stress in my life, and I'm in better physical shape. How can you put a price on that?
Yes, everyone's situation is very different, I consider myself very fortunate, but then again I brought about my current situation by actively choosing to create this lifestyle.