Issue: passenger side headlight lens is cracked and dealer claims that crack caused by impact. need to replace. Estimates for of replacing have ranged from $250 to $800 (the dealer is the $800)

Questions:
1. Is this a prevalent problem, and is there any way to tell wether from impact as opposed to stress. (This car still under warranty)
2. Precious postings indicate that a 2000 model can be as little as $80 for the lens and 10 mins to replace. I could not find equivalent postings for 2001 model which appears to have a different glass that is more heat resistant, and it is claimed, one must remove the bumper to replace - therefore the high cost. Any feedback on this?