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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.

Summary:
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.