I don't agree here. If Tomahawk cruise missile fly to Russia, than it most probably have nuclear warhead. So if no other SAM in IADS is in better position, than firing an S-500 missile against it is still OK as S-500 missile is still far cheaper than destroyed town.
Yes... it is not about trading dollar for dollar in missiles, the threat missiles are intended to hit targets and the dollar value of those targets needs to be taken into account... not firing a 1 million dollar S-500 because it is only a 1 million dollar Tomahawk and you don't want to trade dollar for dollar could result in that million dollar missile destroying Moscow or Leningrad and doing trillions of dollars damage.
Obviously there wont be hundreds of thousands of S-500s made and important targets would include likely ICBMs and indeed in late model versions of the S-500 GPS satellites and other important satellites too.
In many ways it is a bit like small arms fire and artillery... you see a suicide bomber from 2km distance running towards you... there might be a guy with an AK-74 1.8km away from you who could take him out with a round that costs a few cents, or you can get the sniper sitting next to you to take the shot with a 14.5mm HMG with a couple of hundred dollars worth of HMG rounds, or you could have a nearby METIS-M1 team fire a missile worth a few thousand dollars and destroy the suicide bomber and the car he is in.
There would be a lot of factors involved including what the suicide bomber is doing... they might have noticed the nearby troops and they might be targeting them so launching a Metis would take too long to impact to be the option of choice. Equally they might not realise there are troops so close and they might be going for you... either way I would say a spray of small arms fire from the troops in close would be the cheapest and fastest response.
Waiting for the vehicle to get closer will also reduce the shot to impact delay for the other weapons and make a kill more likely... but even at 1km a burst from 2-3 PKMs or PKPs could probably do the job too which would save the long range sniper ammo and short to medium range anti armour ammo for some something that requires that sort of munition.
A good IADS means selection of the right system to engage. In previous wars they probably wouldn't have known what was happening but the small arms in close probably would have taken out the target anyway.