Smart interceptors (like Iron Dome, today) are always going to be way more expensive (like $90k) than the incoming warhead (Palestinian Shahab, like $300).
One thing the Israeli's do is only expend an Iron Dome if and only if the incoming Kasam or Shahab is heading to a populated area. When the adversary is going as cheap as these Hamas rockets are, they have pretty poor accuracy and frequently land in empty areas.
There's no sense in being precise when you don't even know what you're talking about. -- John von Neumann