Ever heard the expression "two wrongs don't make a right"?
...plus you're talking about
"OPAL" SSDs (an enhanced security specification) which will inevitably carry a premium (
maybe the Mac's encryption is theoretically equivalent, but that's irrelevant if you are obliged to meet some security certification standard). Even then, the 256-to-512GB option is $36 cheaper than Apple's 256-512 upgrade. Part of the beef with Apple is not just the cost of the upgrades, but that they low-ball the specs on the base model.
All of the $1000-$1500 Dell XPS 13s, for example come with 512GB SSD, some with 16GB RAM. Look beyond Lenovo (who are probably still costing on the echoes of "nobody ever got fired for buying IBM") and 1TB SSD is pretty common on any laptop over about £1500. Those are
not "tech nerd" options - they're fully assembled, mass market products - and 'BTO' RAM and SSD upgrades are still around half the price of Apple (someone already posted Dell prices).
Yes, some of the more extravagant claims made here come from comparing cheap SATA M.2 blades. However, even high-performance NVME x4 blades still cost substantially less
retail (i.e. someone's getting a profit) than Apple's prices.
Apple's prices have little to do with bill-of-materials costs: they want $200 for an extra 8GB of RAM whether it's bog standard DDR4 SODIMMs for an Intel Mac Mini or a whole new SoC with LPDDR5 chips. Apple are a huge consumer of RAM and SSD and will be able to negotiate the best prices - they don't seem inclined to pass those savings on.