I just went through this process in early April. In Washington state the insurance company must "make you whole". In layman terms that means return you to the condition you were in before someone else changed you.
For determining value of a car they were required to find the selling price of a vehicle as similar as possible to your totalled one. (side note: if they fix your car you can also collect for the decreased value of a previously crashed vehicle.) My insurance company had to find "comparables" for sale. For my 1990 560 SEC this was not as easy as they wanted. Their search is usually within 50 miles of the insureds residence. For mine they found one in Portland Or, (180 miles) Boise ID (400 miles) and Anchorage AK (somewhere past the curve of the earth) Interestingly they missed one in Spokane which was priced much higher than the others. (a fellow lister alerted me to that one)
They then use these prices to determine what a willing buyer would pay for the car from a willing seller, and determine what your car is worth. It is important to also note the items on your car which increase its value. CD player, new tires, and impeccable cleanliness added to my value. You can request the appraisers report. I found that my engine compartment was "average" which means "dirty/dusty, belts/access some wear, oil/fluid leaks possible." You could eat off my engine. In fact in the collision the hood was crumpled and unopenable and the appraiser probably never even looked at the engine.
My tires were rated "good". Good condition is "30-70%, or 4-7/32 of tread left." I produced the sales reciept for the tires which documented 3004 miles on 80,000 mile tires. They paid the full price of the tires in addition to the car value.
They also tried the "average door dings and paint wear". Since 'only' the front and rear of mine was crumpled I challenged them to show me the door dings and paint wear. (there were none). Then they offered excellent paint condition.
I guess my point is the insurance company will value at the low end of every field. You must prove or maintain that the value of your car was better than their valuation. Bring receipts, pictures, and other documentation that your car is better than average.
A great source of information is your state insurance commissioners office. Call them and ask specific questions on items that don't seem right to you.
Three times now that I've been involved in accidents the insurance tells me they will only pay for a subcompact rental car. "We only have to provide you transportation". That is a lie in Washington state. Ask them to put that in writing that they will only pay that amount. They won't do it. State law requires that they put you in a "comparable car". The insurance commissioners office tells me that I probably would have a fight for a new MB loaner, but a full size luxury car (caddy, lincoln, etc.) certainly falls under the comparable definition for an S class MB.
Don't take the insurance company's word for anything. Don't be afraid to have them put everything in writing. They are famous for "no, we didn't say that". Keep copies of everything. Copies of any statements, copies of any forms etc. My company "lost" my file and needed to recreate it. Could I please fill out these forms again? No, I won't fill out the forms again but I will give you copies of the originals. You could hear their face fall when they found out I have copies of everything. I guess they won't be able to show I changed my story. I know exactly what I wrote the first time.
Personal injury? Keep a diary. write down what hurts each day. Amazingly one part hurts one day and another part the next. Document this. Keep copies of Dr. bills, the insurance company will offer pain and suffering money based on the amount of Dr. bills. Sometimes they "lose" some of the bills. (yes I have proof, it was on another persons desk and we just didn't see it.)
The insurance adjusters are overworked, underpaid and sometimes make honest errors. Some people try to scam the insurance companies and fake claims. If you are the honest victim you are responsible to make sure that their mistakes don't cost you money.