This is not exactly correct. If you have an M5 Pro chip instead of m5 Chip - I just built a 16inch, M5 Pro chip, it is $400 to go from 24 -> 48gb. An additional $200 ($600 over base) to go to 64gb. So the memory prices change based on chip. M5 Max Chip starts with 48gb of memory.

M5 Max starts at 36GB memory at $3599. M4 Max started at the same memory at $3199. They have doubled the default storage from 1TB to 2TB, that's a $400 increase I'm paying even if I don't want the extra 1TB.