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
If you still have possession of the rented car, and if you didn't purchase the optional "everything's completely covered" insurance, and if it's still safely driveable, and if the pre-existing damage was noted on your agreement to rent the car (at a pre-rental inspection)...
Then you could take the car to a few accredited (AMA, motor club, etc) repair shops and get some estimates to repair only the damage you're concerned about. Many shops in Canada & the USA will take a quick look at damage & give you a written estimate of repair costs, sometimes for free.
Whether it's driveable or not, take lots of detailed photos of the car, damage and everything (even video too could help). Using photos to get an estimate of repair costs might be possible but isn't perfect, there could be hidden damage not plainly visible, but it's better than nothing.
But once you return the car to the rental company, I don't think there's a lot you can really do, especially without photos. There should be a post-rental inspection, and here your pre-rental photos could be helpful so you're only responsible for the actual damage while you had the car, in case the company's post-rental inspector is more "through" than the pre-rental inspector. But the company can pick whoever they like for repairs, cheap or expensive (probably expensive ;-)
Only options afterwards might be to compare the expenses that the rental compay says it costs, with how much some other local repair shops would charge. But might only have whatever the rental company says was damaged, to ask local shops what they'd charge to fix it.