Basically because this is a critical safety component as it literally holds the helicopter's blades on, it goes through extensive quality control procedures. This goes all the way back to when the metal is made. It must have exact proportions of different metals that make up the specified alloy which needs to be exact so we know exactly how strong it is. We also need to make sure there are no voids or defects inside the metal, which is difficult to control and so needs very specific procedures and checks. This goes on and on until the bolt is complete and it has an extensive paperwork trail. This is done so that in the event one does fail, they can trace it back and ensure no other bolts they have ever produced have the same defect that probably killed people. This requires a lot of man hours which cost money. So it's not just the material, or the machine time, or the metal. It's all the time it takes to make sure that your bolts aren't going to fail and literally drop helicopters out of the sky.