Comparing the price of expansion storage in the PS5, a 2280 nvme drive, to the Xbox which uses a special drive around the size of a 2230 or 2242 nvme isn't really playing fair. The only 4tb 2230 or 2242 I could find was around $400 so the Xbox solution is fairly priced.
All that being said, Microsoft should have gone the Sony route and used a standard 2280 nvme over a customized drive.