How much is a monthly ferry pass between Sausalito/Larkspur and San Francisco?

I've looked at the GG Transit and Ferry website and there is nothing specific about a monthly pass. Does this mean it doesn't exist? We are looking at moving to Marin County and would like an idea of how much the ferry would cost us per month if we will be commuting to downtown SF. Any info would be great, and I already have the link to the GG T&F website, so specific answers only would be awesome. Thanks everyone!