I don't think you should use Apple to put upgrade pricing in perspective. They've used inflated pricing to extract extra money from a captive audience for many years. Other OEMs cannot demand similar premiums for commodity products.
They use on-SoC RAM chips, essentially a cheaper HBM instead of just some random SO-DIMMS.
I think a better comparison would be Dell or Lenovo who do the same thing HP does, and they too charge a lot for what are essentially OEM versions of Samsung and Hynix memory anyone can get.