Having spent a few years "doing battle" with insurance guys what Denny and Richard suggest is a good start. Have the appraiser value the car in pre-accident condition. As a typical "rule of thumb" insurance companies will "total" a car when repair costs are estimated at approximately 80% of full determined value, the rest is considered salvage value for the good, salvagable components on the rig. As additional ammunition the survey of Hemmings, or any of dozens of online sale sites, including ebay, will give you multiple examples of comparable values. Be aware though, with the exception of ebay, all the others are ASKING prices, not sales prices, which are more representative of market value.

Even if you do all that research and pay for your own appraisal, the insurance folks may try to blow that info off, or buffalo you. They may still undervalue your vehicle (assuming you've done an honest job of finding a value rather than trying to fool yourself). They don't make money by being generous. You have another avenue of appeal. Your state probably has an Insurance Commissioner. Their job is to "license" insurance companies to do business in your state. If an insurance company gets a complaint through the Insurance Commissioner's office, they usually respond quickly and more fairly as too many complaints in their file could jeopardize their ability to do business in the state. You have to demonstrate to the Commissioner's office that you've tried to work it out with the company first, so this isn't your first avenue of effort. This is when having documentation, as much as possible (he who has the thickest file wins in these kinds of deals) will work for you. Just going in with your opinion against theirs (remember, they're the "professionals") isn't to your advantage.

If they are close, but the value is still too low for them to justify repairing it you have another option. You can take their settlement amount, pay them the estimated salvage value (in essence buy back your rig) and then spend whatever extra amount is necessary to fully return it to pre-accident conditon. The drawback to this process is you'll likely have to surrender your existing title and request a re-issue from the state, at which time they'll give you a title marked "salvage" or "salvage recovery" or some such terminology. This title "branding" usually diminishes the value in the future since the next buyer will be suspicious of the repair quality and will either kill the deal, or expect to pay some lesser amount to compensate for unknown concerns.