There is no "typical premium" number. The prices for new construction are usually subjective and one can not easily compute a factor for the difference. It's best to specifically analyze the property value. Let me know if you would like to explore that further.
Marin Properties: http://www.mlsfinder.com/ca_bareis/oggikashi/
Broker Associate, Paragon Real Estate Group CA DRE 01844627
All data from sources deemed reliable but subject to error and omission, and not warranted.