http://www.airportus.com/airport-shuttle-taxi/

San Francisco Airport to Union Square – Taxi or Lorrie’s Shuttle Service?
What is the better solution to get from San Francisco Airport to Union Square?
a) Taxi
b) Go Lorrie’s Shuttle Service
I’m asking because my hotel is offering this service. Go Lorrie’s Airport Shuttle cost 39 $. How much would a Taxi cost? Where is the difference?
http://gosfovan.hudsonltd.net/res?USERIDENTRY=UNION&LOGON=GO
a) Taxi will start at about $45 depending on time of day and traffic conditions.
b) Lorrie’s is an ok shuttle service but $39 one way is very high. You can get other shuttles to your hotel for about $20 though you may have more than one stop before reaching your hotel. If the Lorrie’s shuttle price is to and from your hotel it is a good value and you would probably have direct service to your hotel.
As others have suggested BART is the most cost effective way to get from SFO to Union Square the fare would be $8.10 one way
http://www.bart.gov/tickets/calculator/
ESL Lesson: Hiring a Taxi, Limo, or Shuttle from the Airport: