Google it, mate. You or I could place an order today for $19/20 a unit of 8GB in bulk. The exact same stuff Apple use. Don't you suspect they get a bigger discount than that? Or were you under the illusion their RAM was somehow unique?
They could socket it if they wished. Again, Google is your friend. The speed and energy differences are tiny. A year ago this wasn't an option, I'll give you that. They won't socket it again, though, because it wouldn't benefit them, as it saves a couple of dollars and their business model revolves around upselling RAM and storage. Storage, for example, does not benefit from being soldered. That's simply to lock us in, and save Apple approximately $1 on build costs.