It's not your car, it's theirs and they are entitled to have it look as good as it did when you rented it. Ultimately, the car is going to be sold and Enterprise should not take the hit for loss of value on account of the damage for which you are responsible.

When was the last time that you priced refinishing a bumper at a body shop? A couple of years ago, I was quoted $480 - $550 after my college student's car was scraped by another driver (only the bumper cover was damaged - nothing else). My estimate did not include the loss of use of the vehicle for the rental company.