Maybe the best anti drone would be a swarm of drones you send off to crash into another drone? You could model the schooling from fish or birds, then just have enough of them fill the air with enough density to guarantee a hit of the target within the error of the range finding technology.
The problem is logistics. For this solution to work, the counter swarm should be more capable than the incoming swarm (faster, more AI capabilities, etc), and their quantity should be more than the incoming swarm.
You cannot have millions of drones everywhere, but the enemy just has to be lucky and send a million drones to attack you.
Also, you can have advanced tactics, like releasing the swarm, but programming them to hide and attack at random. With a good enough drone, you can create a almost impossible to evade booby-trap. Especially if you are in a war field.
Counter drones like anti-missile missiles need to be bigger than one would think and require much more energy, because they need to be much faster and more maneuverable than the drones they are hitting. That means bigger motors, higher discharge batteries, and a stronger airframe.
I'm fairly certain they would be more expensive than the attackers drones, and you would need around two orders of magnitude more of them than the attacker.
