Injection Molding is a manufacturing process that can easily produce parts in large volumes. It is typically used in mass-production processes where the same part is being created thousands or even millions of times in succession using a mold.
Once the initial costs have been taken care of (mold fee and design) the price per unit during injection molded manufacturing is extremely low. The price also tends to drop drastically as more parts are produced. Injection Molding also produces low scrap rates relative to traditional manufacturing processes like CNC machining which cut away percentages of an original plastic block or sheet.