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.
Money isn't everything -- but it's a long way ahead of what comes next. -- Sir Edmond Stockdale