PCIe 4.0 is going to be pretty standard tech, I wouldn't neccessarily equate their cost with being anywhere close to what it costs to make this stuff.
SD card commission will charge a high price at first likely because the only type of person who needs this much speed are video professionals are who are shooting like 5K-8K raw video files. For them a couple of hundred bucks is nothing.
But that doesn't really mean it costs them that much more to make.
Nintendo could probably get a cartridge slot for example with similar speed probably for a reasonable cost in a few years.
That's understood, and it was a smart move to couple SD cards to PCI-E lines.
It's just that the producers want to make money from new tech, and thus sell at a high price, especially early on. It will take years until the competition has heated up enough to bring the prices down to a level where SD Express will become a viable choice over the older UHS standards. And like I said, even there UHS-I is still by far and wide the most widespread despite the tech being 10 years old by now and UHS-II being just a year younger yet still very rare. At least the pre-UHS models are slowly dying out in industrialized countries (still saw tons of 2-8GB SDHC cards in the Philippines, so in developing countries, it's still another matter), so they are slowly moving forward.
The Nintendo eShop rating Thread: http://gamrconnect.vgchartz.com/thread.php?id=237454