Below 100 feet it is remarkably difficult to detect and stop a drone in time to defend an attack. Especially since they can be fully automated, have onboard inertial measuring units and computer vision. Your best defense would be hunter killer drones over high value targets or trained birds. But defense always lags offense.
Guided missiles used to be an expensive proposition. Now they are off the shelf parts for a couple thousand on digikey.
And again, I think if someone built their own drone from parts, the surveillance drag net would be able to trace down purchases.