[asterius]

You are correct, much better lift, speed, economy from a single large slower moving rotor (or a pair of counter-rotating blades, cost/benefits described in other posts).

The reason all the cheap drones use 4 or more engines is that they can individually be very simple, with the motor directly driving a rotor. With no swashplate to alter blade pitch the mechanism is very simple. Short blades are easy to make for a required strength and stiffness, and the torque of electric motors is sufficient to rapidly change rotor speed to change lift for steering. The control system is simple. However efficiency is terrible, both in terms of lift, and maneuvering.

The energy to change blade speeds constantly to maintain attitude is a significant cost. But having low rotational inertia, frequent changes are required, especially in windy conditions. As blade diameter increases (and hence inertia) the resulting energy costs to change speed increase too, as does the size/power/weight of the motor.