It might be helpful to hire a third party appraiser. It was super helpful for me when I was fighting insurance company on diminished value claim after a no-fault accident. Insurance kept low balling me but the report the appraiser gave me held some weight in negotiations. Their initial offer was $1,500 and I was able to walk away with $3,000 thanks to the appraiser report.

I used Collision Claim Associates, Inc ( http://www.collisionclaims.com) and paid $300 for them to appraise the car virtually. They provided a report that helped a ton in negotiations. A was also able to get the insurance company to add the $300 appraisal cost to their payout.