I agree the price would be a little high but the price of the S-350 will be high too. S-400 is almost 500 millions (price of a big frigate).
S-350 batteries would be less obvious and could stretch across the entire border... not just areas where there is water...
And you could move them around your territory as needed.
You can make the corvette cheaper by taking of the guns and the armour by building it to civilian standards.
You can make it cheaper by making it useless for anything but SAM picket, but why waste money on a ship that sits and waits for someone to attack Russia with lots of cruise missiles?
If they were fitted only with SAMs then a torpedo to sink them all just before NATO launches its cruise missile attack is probably what I would do and all that time and money is wasted.
In reality 200 missiles will be flying to their targets and you will have to refuel and rearm all of your fighter before lunching them.
NATO forces are watched 24/7... if they started moving cruise missile platforms close to Russia the Russians would likely notice.
There are ADF aircraft sitting ready armed and fuelled in case they are needed ready to take off... that is what the old PVO did.
The new ADF will be doing the same with long range radars looking for low flying threats...
Then they have to detect the missiles and shot all of them (they won't be flying togather you will have to look for each one).
Unless they use 200 separate platforms to launch the 200 missiles they wont actually be that far apart. They wont just fly directly to their targets either, which means they will route them the way they think will allow the most of them to get to their targets unseen, which will channel their flight paths considerably. Firing one and then firing one a half hour later would not be very effective... most targets will need several missiles to get any chance of a hit so these 200 missiles will all be on the move at the same time and most will be going to targets other missiles are attacking too.
As I said before a single group of 4 MiG-31s will each have 4 x R-33s and likely 4 R-73s all of which would be very effective against low flying cruise missiles and if attacked by 200 missiles it is more likely they will send 50 MiG-31s and not 4...
The likely targets those cruise missiles are flying towards will likely have their own ground based air defence forces too, and the sheer number of major SAM platforms in Russia means the odds of flying near one is very very high.
The time needed to get an AWACS aircraft airborne and light up all the aircraft and launch AF aircraft... I think you will find 200 cruise missiles is not nearly enough.
While with 20 corvettes, you can make a line where every flying target will must go through and your radars which are more powerfull than fighter radars will detect all of them. Like I said you can keep S-300/400 onthe coast for f-35/ but for cruise missile cheap Tor and Shtill are enough.
For cruise missiles even Igla-S is enough...
If you let the enemy go in your backyard hoping that you will shoot him down when he is there is non-sense.
Building 20 dedicated SAM picket corvettes to defend the very few cruise missiles that might actually come via a sea border is a bit silly.