Edit: I just took a look through the warranty policy and couldn't see anything about your warranty being voided for cosmetic damage, only not covered. If the iPad had a problem that didn't arise from the dent/drop, wouldn't Apple be obligated to fix it? Regardless, fair price?
if im seeing that correctly, i have a corner metal ding like that on mine too and i have no idea how it happened- its always been in a case and inside the house. maybe it fell off my bed?