Generally the value is decided by the salvage yard or the person who is purchasing the car. That means it's only worth what someone is willing to pay for it.
Insurance companies usually have a set rate such as $200 for totaled vehicles that they had to pay a claim for.
If the owner deciides to part out the vehicle - then it could be worth considerably more but it will take longer to sell the parts.
Craigslist and Ebay have many listings for parts from totaled vehicles.