Let’s see if this article from Insider makes any sense?
送貨成本計算：（能源成本 + 操作人員成本 + 載具費用攤提）
這個基本公式，不論用卡車或是無人機，都一體適用。從Amazon日漸廣佈的fulfillment center送貨到客戶那兒，Prime二天、Prime Now二小時、再進到將來Prime Air以半小時為目標；其同時、同路線的出貨數量都趨於靈活之小量，大型貨車根本使不上力。用無人機，會比起用卡車的單趟人力貴，我都高度懷疑了，還一萬倍？
送貨的能源成本：卡車 vs. 無人機
每英哩，卡車是 $0.5 而無人機只要 $0.02。所以，混用卡車和無人機來完成送貨到府，就能大幅降低亞馬遜必須支出的運費
the fuel cost of running a diesel-powered truck is a little over 50 cents per mile, whereas the energy cost to power a drone over that distance is about 2 cents. Combining the two vehicles holds the potential to substantially reduce the cost inefficiencies associated with “last mile” delivery.
在亞馬遜遞送給消費者的商品包裹中，有 86% 輕於無人機所能載得動的重量（2.3 公斤）
The drones … can carry packages weighing up to five pounds (2.3kg), which make up 86% of the packages Amazon delivers.
Amazon has written that 86 percent of its packages weigh under 5 pounds. As for distance, Walmart has noted that 70 percent of Americans live within 5 miles of a Walmart. This is great for the prospect of drone delivery.
In a report by ARK Invest, Tasha Keeney suggests that Prime Air could cost Amazon only 88 cents per delivery. If Amazon charged customers $1 per delivery, Keeney estimates, the company could earn a 50% return on its investment in drone infrastructure while offering same-day delivery that is significantly cheaper than current alternatives.