Most likely, you and he should split the cost. You would be considered to have been liable due to negligence, or unreasonable carelessness, in running over an item (the bag) in your driveway; running over a stationary object in your path is generally negligent. However, your negligence is offset or reduced, under the doctrine of comparative negligence, by his negligence, or carelessness, in leaving the bag someplace he should not have left it. While it's impossible to say with certainty what a court would do if it came before the court, one likely outcome would be to balance the faults so that you each pay part of the cost.