My understanding is that the LPDDR4x chips cost less per GB than the random chips you find in the common dimms. There's also costs (board space, part cost, motherboard layers, and layout complexity) for dimm slots.
Sure manufacturers might try to charge significantly more than market price for on the motherboard RAM, but it's an opportunity to increase their profit margin and ASP. Random 2x16GB dimms on newegg cost $150 per 32GB. Apparently LPDDR are easier to route to, require less power, and cost less for the same amount of ram. I'd happy pay $500 for a motherboard with 64GB of LPDDR4x-4266. Seems like Asus, Gigabyte, Tyan, Supermicro and friends would MUCH rather sell a $500 motherboard with ram than a $150 motherboard without.
Normal rate ( Not Contract Price ) for LPDDR4 / LPDDR4X and LPDDR5 is roughly double the cost of DRAM per GB. Depending on Channels and package, the one used in M1 is likely even more expensive as they fit 4 channel per chip. DIMM and Board Space adds very little to the Total BOM.
