Why is single channel RAM cheaper? I thought the most expensive part of RAM is fast, high-density chips.
It is.
The main reason for it as stated in previous Valve interviews was due to lack of component supply not pricing so much.
Adding the 2nd RAM stick and enabling dual-channel will not fix its only 8 GB of VRAM, as the GPU part is not upgradable. Just get a PC and install SteamOS. One might call the 8 GB of VRAM VALVE's strategic mistake (look, even the big reviewers on YouTube complain about the only 8 GB of VRAM and they have millions). The 8 GB of VRAM issue started like 4-5 years ago.
Here is a more recent one about only 8 GB of VRAM: youtube.com/watch?v=ric7yb1vaoa ("gaming laptops are in trouble - vram testing w/ ‪@hardwareunboxed‬").
Indeed, the recent memory price increase doesn't help this issue.