Those gunships are rather vulnerable. Iran just might have the stuff to take one down.
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Assuming someone can see them from 10 miles away and 3 miles high. Russia would have to give the Iranians several dozen S400 systems with 20 linked tells each to cover the territory the proxies operate in.
Right now Russia is hurting for S400s ($20 billion each) and most are around Moscow at the moment.
Vulnerable? Think not. They have more then just a cannon and a howitzer, by the way. Not to mention their fighter escorts ...
$20 billion for one set of missiles (How many?) & launcher system each? Seems a bit steep for a Russian system.