There's a ton of ram types with varying performance levels, and in apple's case it's RAM with direct and performant access from the graphics card (unified memory).
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.
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 cost to Apple for going from 16GB to 32GB is less than $40. So of course it's priced at $400.