The SSDs they use really do cost that much though. Even if you got them from another manufacturer they'd be about that expensive. Apple don't use cheap SSDs...they use blazing fast, top of the line units not found on many other laptops.
If you have criticism, price isn't a valid one.
But you can definitely argue that they should have cheaper/slower options so that consumers who don't need blazing fast SSDs can still benefit from increased storage.
Here's how the 2018 Macbook's SSD stacks up against the competition: https://cdn.macrumors.com/article-new/2018/07/macbookprossdt...
That's why there's such a huge price difference. Not because they're randomly picking high upgrade prices to screw customers.