Using a $2,000,000 missile to bring down a $200,000 drone is unsustainable.

Depends on what the drone is heading for. If it's heading for an apartment building that is only worth a few Mill, maybe not. If the drone is heading for a $13B aircraft carrier, the economics of shooting down the drone look a lot better.

What ever happened to "Metalstorm"? Ah, Wiki says went defunct in 2012.

en.wikipedia.org - Metal Storm

During WWII, the Japanese developed a "beehive" round for their battleship guns, that turned them into a blunderbuss.

San-shiki-dan (三式弾; "Type 3 shell") was a World War II-era combined shrapnel and incendiary anti-aircraft round used by the Imperial Japanese Navy. They were supposedly referred to as Beehive rounds. The shells were intended to create a large volume of flame which attacking aircraft would have to fly through. However, U.S. pilots considered these shells to be more of a pyrotechnics display than an effective anti-aircraft weapon.[A 1]

en.wikipedia.org - San Shiki(anti-aircraft_shell)

Toward the end of WWII, the USN developed autoloading 6" and 8" guns that could fire 10-12 rounds per minute. Load them up with flak rounds and they could put up a fair bit of shrapnel that drones would be flying into.

Combine firepower with cheap, chaff rockets, and flares, and a self contained drone would be pretty confused and skinned up.

Steve