SoCalGirl is probably right, this looks like an argument you can put together yourself with a little research and investigation.
1. Make a full assessment of the state your car was in before the accident. Write down everything you can remember. (1) mileage, (2) prior damage, and (3) wear and tear, including any rust, are all essential.
2. Take the information you've got and plug it into all the auto valuation calculators you can find. I checked CCC (
http://www.cccis.com) to see if they had anything online and they didn't appear to, but the National Automobile Dealers Association has its used car guides available (
http://www.nadaguides.com), and the Kelley Blue Book (
http://www.kbb.com) is pretty well known, also.
3. If you're skeptical about the numbers you see, then do a little research and see what similar cars are actually being sold for in the area. This is not the same as what they're being -offered- for. Classified ads and dealers tend to advertise well over the price that you would expect to pay! The sale price, not the advertised price, is what counts. Unfortunately, it's harder to find.
4. If, according to your research, the number is still out of whack, write the insurance company a letter explaining everything about the car that you had and detailing why your research shows that their offer is inadequate. You are generally entitled to receive enough money to replace your car in your local market, in the condition that it was in at the time that you lost it. You are not entitled to receive enough replace it with a vehicle that is newer or in better condition.
Good luck!
Oh, I have to add the usual disclaimer. I'm not a lawyer, do not know insurance law, and you cannot rely on what I have said for legal advice. I'm only recommending ways that you might assemble a strong argument to make your case to the insurance company.