In the case of disk-based media, the primary cost is the moving parts inside the drive. This sets a fixed lower limit, which is why the average selling price for both of the major HDD manufacturers has been US$45–75 since 2007. That said, the price of high-capacity drives has fallen rapidly, and this is indeed an effect of density. The highest capacity drives use more platters, essentially individual hard drives within the case. As the density increases, the number of platters can be reduced, leading to lower costs.