I’d much rather have had serviceable ram modules than sightly faster ram to the gpu I will never fully flex with macos software anyhow. Speaking as someone saddled with one of these computers.

Every M-series Mac has shipped with completely standard LPDDR4 or LPDDR5(X) memory chips. While these can be a bit more expensive than socketed non-LP DDR DIMMS, we're talking maybe 10% more expensive, not 1000% more expensive (which is what Apple charges for upgrades vs standard retail price for DIMMs).

Apple's marketing department would be happy for you to think otherwise, but the "secret sauce" of their high memory bandwidth is completely due to having more memory channels built into the SoC than a standard x86 CPU.

RAM is literally a JEDEC standard.

You're not buying a standard, you're buying a module with particular packaging and specifications, all of which have a huge impact on production cost.

RAM cost is predominantly the cost to produce the silicon, which is a standard and interchangeable. Apple uses multiple sourcing as well to lower their cost.

Isn't the memory in the Surface Laptop 7 (and presumably other comparable Qualcomm/x86 laptops) quite a bit faster?

Of course MS overcharges on upgrades as well... because they can. Can it really have anything to do with cost, though? I wouldn't be surprised that the slotted RAM in laptops that support it and which Lenovo/Dell/etc. sell pretty cheap ir as or more expensive than the "Apple Magic Unified Max Marketing RAM" wholesale?

The fancy part is how it is attached. The memory modules themselves are one of the cheapest kinds of commodity ram.

The cost to Apple for going from 16GB to 32GB is less than $40. So of course it's priced at $400.