1. It's more like $45,000.per interception.
2. A Grad missile is hardly a "$15 flying gutter", it contians a good amount of TNT and costs at least $100
3. During operation Cast Lead alone, at least 10 Israelis died or were seriously hurt by such Grads.
4. The damage to property by those Grads greatly exceeds the cost of interception.
5. The moral boost to people, who don't have to witness missiles falling around them, is even more important than the sheer economic value.
Your demoralization effort is pretty lame, honestly. Why don't you get a normal life?