Body piece Prices?

You can usually just LOOK under the car and see if there is any damage to whatever is behind the bumper.I would assume they remove the bumper to fix it, most come off in less than an hour, and it allows you to put it on a bench for repair and painting.
This is the best way to get the average of the price of all the body parts.

call a local salvage yards to get prices. you could get the door, mirror and front fender for less than $500 from a salvage yard. but the rear quarter is most likely $500+ thats something a body shop would have to do because it would have to be spot welded back on. google prestige mustang parts or call them at 404-294-4869 they have tons of 99-04 mustangs that they sell parts off of. they are pretty reasonable.

Do you have collision insurance?