As range of a missile interceptor is over 250km, and if they have 200 missile points at the distance of 50 km each, it will cover 10,000 km border, which would include Indian costal area also. and if they put 10 missiles at one missile point, then they will need to buy only 2,000 missiles for BMD for the price of 2mil to 3mil each (with total price of $4bn to $6bn for 2,000 piece + $2bn more for establishing infrastructure) to have a missile defence like how US want to do with EU, to cover whole indian territory.
And if India may try to have two BMD layers, One by India itself around its main cities and one this russian S-500 ABM system on border, hopefully, then even if the missile defence on border miss any incoming missile, the next one around the cities will have a better idea about that incoming missile, I think
Unit cost of arrow-3 is also around $3mil
Arrow (missile) - Wikipedia, the free encyclopedia