Ironically, one of the best defenses against drone swarms is likely to be what the US defense industry has been terrible at for decades, which is gun-based air defense. There are medium (35-40mm) autocannon shells now that can have time fuses programmed as they're being fired based on the measured velocity of the round (making it more precise than average muzzle velocity) and the radar range to the target. They're still more expensive than a conventional round, but Rheinmetall was selling 35mm shells of that type for ~$600 each last year, and the price will likely come down (in real terms if not necessarily in nominal terms) as the technology matures. The exchange rate shifts a bit if you're downing drones with a 5-round burst of gunfire at a cost of $3,000 instead of using a million-dollar missile.
The sub-30mm currently aren't seen as being as useful for C-UAS work because the lethal radius of their airburst rounds is much smaller.
The other thing being worked on is electronically frying drones with microwaves, such as the
Leonidas system (there's also Phaser, THOR, and CHIMERA from other manufacturers). Basically, your small cheap drone is going to have pretty much no radiation shielding, because that adds cost and weight. A sufficiently intense blast of microwaves will disrupt the electronics and shut down the drone rather than just disrupt its data link. The capital investment is pretty high, but the cost-per-shot is extremely low, and range is in the realm of 5-10 kilometers. A layered defense with high-energy platforms as the first layer with gun-based air defenses as an inner layer to mop up any survivors would likely limit the effectiveness of the cheap swarm attack, although exactly how effective the defense would be is a fair question.