It’s telling that Syria/Russia apparently did not deploy the S-400 during the missile attack. At $200m per system, I don’t blame them.
It's not so much the cost as it is that Russia simply does not have enough of them or enough manufacturing capability to make enough of 'em, and we have thousands of Tomahawks to throw at 'em. They would simply run out of ammo in the middle of a war.
But that will eventually change.