The numbers I've seen are that the interceptor missiles cost anywhere from $45K to $70K apiece (seen varying numbers). The rockets being fired are about $1K apiece, and they have thousands.
The fundamental idea of this Iron Dome system and others like it is flawed - especially in economic terms. Spend $45K-$70K to shoot down every $1K rocket? That can't last long before you're utterly broke. This is economic FAIL. An alternative solution is required.
This is a classic example of asymmetric warfare.