Drone isn't necessarily a quadcopter. It can be an RC plane, too, and those can be cheap in larger sizes, although they are more difficult to fly accurately. I'm actually surprised this isn't more prevalent - any teenager could do it, if they had access to explosives, which teenagers in war zones (current or past) typically do. And so do adults, of course.

Truly we are blessed with unequal distribution of technical skill between the terrorists and the rest of us. If a real, world class engineer took up terrorism, they'd never be found and could do _a lot_ of damage.