Yep, that's a ripoff but expansion cards always are. Gotta make that money somewhere.
It's about what a bare. no-casing PCIe Gen 4 nvme drive costs. It's far from a rip-off. Best case scenario for a similar non-proprietary drive is $20 cheaper.
No cost yet since they're not going to be proprietary and are only based on QA certification from Sony. But given the price range for these is $200-230? It'll be around the same price.
Nope. NVMEs with similar performance to Sony's built-in solution, and the newest ones with faster speeds are around the same price. Sony's not doing proprietary, so the cost would be similar to this, not higher.