Injection molds are made up of special, high quality materials that have a good machinability property. Some standard mold components such as springs, bolts,

and limit switch are prefabricated and costs lower.
Most standard components such as ejector pins, ejector sleeves, and sprue bushings are not prefabricated which mean they will only be manufactured when you order them. They are not mass produced thus they are expensive.
The cavity and core are mold components which make the mold really expensive. It involves careful design engineering and processing. Most of the tools used in making these components are very expensive.