In Seattle, over the last 8 years I've averaged 2933$/year on my car (which I own outright). That includes insurance, licensing fees, parking, gas, maintenance, tickets, rental costs, gps, and tolls.
I don't think you can make a compelling case for a taxi service vs car ownership based on cost -- but rather convenience.
Perhaps not in Seattle. I'm pretty sure the costs you mention are higher in SF (just like everything else...) depending on your circumstances, it can start to approach a no-brainer. Recently, I've probably been spending in the neighborhood of 80 to 100 dollars a weekend on Uber (~$2100-$2600/yr), and the guy above mentioned he spends roughly ~5000/yr on Uber. The cost of owning a car ranges from 5000 to 15000 a year[1][2][3], and it's substantially more expensive in San Francisco because this doesn't include parking, tickets, and tolls, so you can tack on another ~3000 for parking alone, minimum. (at ~250/mo, pretty much minimum)[4]. It doesn't take a lot of imagination to see how for people in certain situations (e.g., no long commute), the cost alone could be more than worth it.
I don't think you can make a compelling case for a taxi service vs car ownership based on cost -- but rather convenience.