New Construction vs. Existing Building

This is actually a question about commercial real estate, but I don't know where to post it other than here.
I have been looking to purchase a small (2000-4000 sq. ft.) commercial building for a business than has outgrown my home office. I have been looking in a depressed neighborhood of Los Angeles County at buildings in good shape as well as fixers. Today, the idea of buying a vacant lot and building on it popped into my mind. All I need is 3000 sq ft. of open unfinished warehouse space, with 1000 sq. ft. divided into three offices. Not even fancy offices. Is there a rule of thumb for comparing the price of new construction with an existing building?
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