It also very often costs the lives of the launching crew. Israel's defense budget is $20B (of which $2.4B is US aid). Israel can VERY EASILY pay $50,000 per Iron Dome launch, for thousands of launches, meanwhile steadily liquidating Qassam launching crews. If in the end it costs half a million dollars to kill one terrorist while saving the lives of civilians in Sderot - that's a very good deal.
One student killed, three wounded at Arizona campus during fight (a)