Another problem is that SD Express needs specific card readers, as they have an additional row of pins. Otherwise they'll only transfer at UHS speeds. And I'm sure that those readers are also still very expensive. Too expensive to use in any console anytime soon.
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.