I'd check out any of the state park historical houses in Monterey. They are all relatively affordable, and some offer really lovely gardens. I went to a wedding at the Old Whaling Station a few years ago and really loved it. Here is the link to the state park options: http://www.parks.ca.gov/?page_id=24737.
Good luck! September will be a beautiful time of year.
We did our wedding at the Old Whaling Station (ceremony there and reception at the Memory Garden). I know that several people do everything at the Old Whaling Station but either venue is great. Good luck.
If you're looking for a garden setting; there are two that I recommend. One is La Mirada; they have a beautiful rose garden and center courtyard area (http://www.montereyart.org/site-rental/) and another would be a new venue currently under renovation but due to open April 2011, called The Perry House in Monterey, you get both bay views and garden views there (http://www.montereyweddingsites.com/perryhouse). As well, for a beautiful, unique and cost effecitve option, -- but with no views -- if you have no more than 80 guests, you should look into the venue called Gatherings in Pacific Grove(http://montereyweddingsites.com).
You can also try Jardines de San Juan in San Juan Bautista. http://www.jardinesrestaurant.com/weddings.html. Think historical town, out of the fog/marine belt, beautiful flowering gardens, brick patio, good value, fun mexican food, walking distance to beautiful historical mission, lots of opportunities for great shots with rustic town and doorways etc in the background. Good luck with your plans!