They're supposed to find a number of cars in newspapers close to your home with your year, make and model and as close as possible to your mileage, and then average the selling prices. Unless the extra equipment was really extra and new, they rarely give much extra, if any, for it. Ask to see their analysis of the value of your car. In the meantime clip any newspaper ads like the above to use in negotiating with them.
If your insurance company is screwing you on the value of your vehicle, your insurance policy probably contains some sort of agreement to take such disputes to arbitration. You would not want to do this yourself - you would want to hire an attorney to represent you.
Sometimes it helps to get an independent appraisal. That would cost about $150. Also, you can get quotes from places like Craigslist and Edmunds to see what similarly equipped vehicles are selling for. The key is to keep papering them with documentation so they have something to present to their supervisors to get you more money. Good luck.
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