The current generation of drones are *much* cheaper than their manned counterparts. When the DoD reports "per unit cost" of drones, most of the time that cost includes multiple aircraft. Per aircraft cost of a Predator B (MQ-9) is ~$10mil. The full reported unit cost of ~$50 million includes *four* aircraft and all the hardware to operate them. Not to mention reduced training, maintenance, and pretty much every other sort of cost.
The Air Force hates using drones, and they resisted widespread adoption for so long, but even they couldn't deny the economics of the platform.