Cost to manufacture: $800
“The utility of the Qassam rocket design is assumed to be ease and speed of manufacture, using common tools and components. To this end, the rockets are propelled by a solid mixture of sugar and potassium nitrate, a common fertilizer. The warhead is filled with smuggled or scavenged TNT and urea nitrate, another common fertilizer.
“The rocket consists of a steel cylinder, containing a rectangular block of the propellant. A steel plate which forms and supports the nozzles is then spot-welded to the base of the cylinder. The warhead consists of a simple metal shell surrounding the explosives, and is triggered by a fuse constructed using a simple firearm cartridge, a spring and a nail.”
From Wikipedia
Perhaps the Israelis should try to ensure the Palestinians only have access to less explosive fertilizer mixtures.