My car can idle and drive slowly, for an hour on less than 1/2 a gallon of gas (have to watch it more closely the next time I'm stuck in traffic trying to get across the Bay bridge). That is $2 in California today. It costs $3 - $5 for an hour of time on some downtown parking meters, can cost $5 per 20 minutes in the parking garages. Trust me, gas is cheaper, and with a hybrid doing the driving, I could see the 'hover' mode being like 50 cents of electricity.
It's only cheaper because the cost of the space you're taking up on the road and the cost of the traffic you're causing is not accounted for. The solution to this is a VMT tax that accounts for that cost.
Perhaps, San Francisco policy is to use parking prices to discourage vehicle use. They haven't gone the London 'congestion tax' route yet, perhaps they will.
There is a difference between what "is" and how the system responds to what "is". In this case I don't doubt for a moment that drivers of self driving cars would use this technique to economic advantage, and yes if it got too bad it would invoke a response (much like the $5 parking meters with credit card readers in them was a response to cars in general).
