Manufacturers have been dealing with NAND flash storage shortage for a while now due to the increasing demand for SSDs and the transition from 2D NAND to 3D NAND memory chips, which can pack in more data. Apple itself has been contributing to the shortage due to the consistently huge number of iPhones it ships. As one of the biggest tech giants in the world, it isn't in as much trouble as smaller manufacturers are. However, it still probably had to pay a premium for memory chips, considering it just launched three new iPhones instead of the usual two.
As 9to5mac noted, Apple CFO Luca Maestri admitted earlier this year that the company has started experiencing "some level of cost pressure on the memory side particularly on NAND and DRAM." We're simply feeling the effects of the shortage that's been going on for months. It's unclear if Apple plans to bring back the Pros' old prices once storage supplies stabilize. In case the additional $50 feels too much, you can always go for the base variants -- Cupertino didn't adjust their prices like it did the other models.
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