The actual cost is difficult to predict, but it likely revolves around the price of the rockets and the fuel used to deliver them. You should also know that Iron Dome is a joint Israeli/US project that also incorporates the U.S Army manned Patriot Battery located in the Sinai Desert. It is safe to say that both of these are vital to protecting Israel from rocket/missle attacks from all directions.
Man shot dead in northern Israel town of Bi'ina, police say the murder was gang related (Haaretz)
from the article: Iron Dome intercepts rocket fired by Gaza militants into southern Israel