The Joint Direct Attack Munition (JDAM) is a guidance kit that converts unguided bombs to precision-guided munitions. There are several reasons why JDAM is one of the most cost effective weapons ever invented.

By using GPS navigation, the JDAM guidance kit can be extremely cheap, as low as $25k/kit.  It is likely cheaper than any other precision-guided weapons.

GPS navigation provides good targeting precision, typically within meters. Combined with blast radius of 500-2000 lbs unguided bombs, JDAM can effectively destroy any targets on the ground.

Because JDAM has navigation capability, pilots no longer need to precisely aim the bombers  against the targets. It allows a bomber to quickly and precisely hit multiple targets along the route.

Some JDAM guidance kits include targeting sensors, such as laser targeting. They can be used against mobile targets, which further increases the use cases for JDAM.

In summary, by combining dumb bombs with a guidance kit, JDAM becomes a very cost effective weapon system, and is widely adopted by the US and other military forces.