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The cost will depend upon how long you intend to rent the car. Take the on ramp to US-101 N. Take that exit and turn right onto Market Street as you get off the freeway. Follow Market Street east until you get to your downtown destination.
Board the blue line on the Air Train, until you reach the Rental Car Center Station. Rent a car. Exit the airport heading west, turn right at the fork. Follow the US-101 North until you see the Octavia/Fell exit.