Comparing to human costs of Amazon's preferred shipping method: USPS's subsidized parcel injection. The last mile belongs to USPS. In the urban environment, that's about 15 minute per parcel at $20 per hour (labor plus trucks). I.e., $5 per parcel. For FedEx, the cost of the entire expedition process is roughly $6.6 per parcel ($43bn cost of revenue for 6.5bn deliveries), so the last mile costs less than that.
Flirtey CEO Matt Sweeny suggests $10 per drone delivery. He doesn't specify from where. I assume he also means the last mile.
Are drones more expensive than humans? Air is expensive. Think of helicopters vs cars. With all congestion, helicopters have not become the vehicle of choice. They're noisy and can land only in certain locations. Most urban areas are too dangerous for them.
Air drones inherit these disadvantages.
As for Amazon, it'd have to stop outsourcing delivery to postal services and build a lot of physical infrastructure. Few tech companies are comfortable with this because they become capital intensive businesses, which they don't like.
So, not Amazon and not air drones. Perhaps ground vehicles within city borders and only after cars learn driving without humans. That's 5-10 more years.