Given two computers:
Has X gigabytes of RAM, that's the amount it ships with and that's all it will ever have, it can be designed around that number in whatever way is most efficient, solder chips directly to the motherboard if that turns out to save money, whatever.
Has X gigabytes of RAM, but field upgradable to 4X gigabytes.
How much does the expandability cost? That is, not the cost of buying the extra RAM at expansion time, but just to include the capability on the motherboard, how much does that add to the initial cost of the machine?
Does it have any effect on performance? If memory wasn't going to be expandable, could you shave cycles off memory access time or anything?