Disagree on always being cheaper. Military planners are obsessed with the best weapons, such interceptors are pricey. But look at Israel: Iron Dome. ~$50k/shot. They deliberately built a dumb SAM because it was designed to go against dumb opponents--objects falling freely on a ballistic trajectory. While they are usually facing light stuff that isn't even worth that they have successfully engaged longer range stuff that costs many times what the interceptor costs.

Overall, though, the offense always wins this one because interceptors can only protect a limited area whereas missiles can go anywhere.