Let's say you have damage to a rental car for which you are liable. Other scratches clearly show that earlier damage was never fixed. How do you calculate the reasonable damage the rental agency can charge you?
I've had plenty experience in dealing with damages to rental vehicles - from completely 'totaling' a vehicle, to dents on the body...all over the world. As stated in the comments, your best bet is to do a thorough walk-through of the vehicle before taking possession.
If you have a credit card issued by a US bank, most offer rental car collision coverage, with no deductible. It is a bit of a hassle sorting through the paperwork with the rental and credit card company, but once done, you will be fully reimbursed for any damage charged by the rental company.
For calculating damages, you will have signed 1 of 2 clauses in the rental agreement:
In case of the latter, given your scenario, the rental company may charge the entire amount of the deductible on your card for any (perceived) damage while the car was in your possession. It is then your prerogative to follow up and get the right amount adjusted. For a few scratches/bodywork, the estimate should likely be less than the deductible.
The bottom line is - it is a painful situation! The rental company will not give you the vehicle to make the repairs yourself. You do not have much recourse unless you have categorically identified the damage before renting the car. Your best option is repeated follow-ups with the rental company ensuring the right repair amount gets adjusted OR they are convinced about the pre-existing damage and waive your liability completely