These guys also rent Minis in San Francisco. I thought maybe they'd be cheaper, but looks like the $85/weekend day is the same, although it's unlimited miles. (They are only $65/day for midweek.) Other interesting stuff too. http://www.cityrentacar.com/exotic-cars.html

I seem to remember a cheap place I found last year when I was checking for something like a Mini to rent. Not sure if City was the one I remember or not. (I couldn't use it because we wanted to one-way to PDX. Just got a Mazda 6 from Hertz.)