EXCLUDING SITE PREP
$300 SQ FT
Of course, you can go cheeper by hiring unlicensed subs and using sub-standard materials.
Site Prep is the killer. What type of reports do have in hand? soils?
Water and electric on site is a nice start.
I'd start with a good local architect (not builder). Talk only to pros with solid experience in LA County. Ask for references and check them.
For $250 -$500 an architect will give you basic idea of the site limitations.
If you decide to pull the trigger ask your architect to refer three GCs.
Be honnest with your GC about budget. Take his advice. Limit change orders. Select and order materials in advance. Pay suppliers on time. Be on site frequently. Time is $$$.
Again, work only with crews who have verifiable experience in LA Co.
Otherwise, you will probably end up getting a lot of conflicting or otherwise non-relevant info from out of the area but nevertheless well meaning agents.
There are indeed lots of factors which could affect the final price per square foot to build, but I'm working with someone who is currently building at $200 per square foot. His plans are interesting and his property is located outside of the jurisdiction of the California Coastal Commission. He is building a modest home with a small quest house. Good luck with your project~!
Unless there are plans and permits included in the price it's best to consult contractors and building consultants for answers to that question.