In the old age and by law of conscience since the Hummer owner didn't do anything wrong, (he just parked his car and suddenly when he came back it was all broken!) then he should not be burdened by any share on the total repair cost, which means if your insurance is only paying a portion then the balance should be paid by you since you cause the damage.

However, modern common sense which is how insurance contracts are drafted works like this:

-The value of original Hummer's Mirror is no longer 100% since it is already depreciated, in this case it's already 50% used
-Since it is damaged and has been replaced with a new one, essentially the mirror which is 50% used has been replaced with a 100% new mirror!
-Common sense says that the Hummer owner even if not at fault should not also benefit from the demise of other.
-Therefore, if he's 50% used mirror will be replaced with a 100% new one, then he should pay for the difference.

It's like, if the Hummer owner simply just wanted to replace his unbroken/undamaged but 50% used mirror, he will sell his old mirror for QR50 and buy a new one for QR100. His net cost is QR50.

As to the person who caused the damaged, your insurance premium is your cost.