>

And most effective - I’d go for this option
Russia makes the bet that Iran can produce drones faster than NATO can produce anti-air missiles. >

Honestly I think they’ll be able to do it cheaper with high volume computer assisted machine gun fire or computer guided anti drone electronic warfare weapons. 
Missiles are expensive but a tracked weapon that either doesn’t need reloading or costs a fraction of a missile is coming. >

cheapest is to kill the source, just need some long range, accurate missiles