Also, "Amazon’s proposed use of drones may drive down the cost to deliver small packages crosstown to as little as $1, a fraction of existing same-day delivery options, according to a 2015 study by New York-based ARK Invest that tried to quantify the savings from the use of drones compared with delivery trucks and couriers."
Most data I've seen does actually suggest that drones won't be cost competitive with delivery trucks anytime soon - esp. across cities. However, speed can indeed be a USP that would be difficult to match with delivery trucks.
Also, "Amazon’s proposed use of drones may drive down the cost to deliver small packages crosstown to as little as $1, a fraction of existing same-day delivery options, according to a 2015 study by New York-based ARK Invest that tried to quantify the savings from the use of drones compared with delivery trucks and couriers."