"Blue Book" Retail price based on year and mileage (The book should be a recognised price guide and each insurance company has their own)
Any additional features that could add value MAY be considered but only if they have been included in the value of the car on your insurance policy before the incident. For example my 1985 sports car has $10,000 in suspension upgrades that I got included into the value. They would be covered. The fancy stereo wont as it is not specifically listed.