What does the rental agreement say? Probably that they will get any repairs needed at your expense. And they have the choice over where to repair the car. It's inevitable that they will have preferred suppliers which will be large, corporate bodies with higher overheads and higher costs than a man-and-a-van outfit. Also they won't attempt to drive a hard bargain as it's not them paying. Inevitably, you'll always lose out. Unless they're really taking the p*** (and it does happen), you probably don't have a case.
You could attempt to say you'll repair it yourself, but they're unlikely to agree (probably too late now anyway). And even if they did, they would no doubt add a huge "inspection/indemnity fee" to make sure the repair is up to scratch, leaving you no better off.
