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.

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

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

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..

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.