I strongly believe that the next step in asymmetric warfare will be the underdogs using swarms of drones where most are cheap, harmless "toys" to turn it into a numbers game to figure out which 1%-10% are full of explosives. As the cost of small drones drop, you'll potentially be able to drive up the cost of counter-measures dramatically with lots of dummies.

The payload is cheap compared to delivery and control.

If you have a type of drone that can carry a useful payload (shaped charge powerful enough to disable a tank, in the ballpark of 5-10 kg, is not that feasible for a small drone - but a something like a frag grenade may be feasible), then you might as make them all real, the dummies would cost almost the same.

The dummies don't need to be a type of drone that can carry a useful payload, that is the point. They just need to sufficiently resemble one for long enough that you have to shoot them down just in case.

There was recent news about drone that could carry full sized movie camera. Price was few thousands for the drone.

If you go handgrenade size, the drone costs just bit over 100$.

A frag grenade costs something like $30 (and that's with military procurement overhead that would also raise the drone price to $300; mass produced third world grenades cost $5-$10 to their armies); a detonator, explosives and ballbearings for a DIY modification to that drone would be much cheaper.

Anti-tank shaped explosive charge would be hundreds of dollars but again, a fraction of the drone cost.

If the drone system is capable of aiming, you might want to try this wrinkle: https://en.wikipedia.org/wiki/Explosively_formed_penetrator

I'm not sure decoys are worth it really. Conventional explosive are cheap. Very cheap. You might as well stuff them into every "thing" you deploy.

It's different with ICBM's.

But yes, I think you have a point in that with COTS robot technology, it should be very cheap to produce swarms of simple missiles. If the adversary answers with "military-grade" countermeasures, he'd go bankrupt..

As someone pointed out elsewhere, though, drones that can carry enough explosives, on the other hand, are not cheap.

But drones that can be made up to look like they are drones that can carry enough explosives when seen from a bit of a distance is another matter.