From what I hear from experienced builders in LA County $250 per square foot in considered reasonable. I'm sure it varies from City to City so you might want to check with local builders.
The Reavis Group
You have been given some good answers. You also want to keep in mind you do not want to over build for the neighborhood. Poway area is nice but you have to know the direct neighborhood home sales before you decide how much to put into it. I have seen people end up $100,000 over budget cuz they get carried away. Think it all through and remember the cost of your patience too :)
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