Repair costs for a scratched bumper?

Today as i was leaving my high school, i had my windows down and it was windy outside and i was waiting to turn out of the parking lot and a plastic bag was about to go out the window so i looked back and grabbed it before it went out the window. as i grabbed it i guess my foot came off of the brake and i slowly bumped into an audi in front of me. my car is fine but the other car has a shallow scratch from my license plate hitting the bumper. How much would a repair like this normally cost? We did not call the cops and are not going through insurance companies as we dont want insurance rates to go up even more.