Specifically, how cheap they are. I was planning on just buying some 3/8" or 1/2" tubing and making one myself, and the raw materials are about as much as they're charging. Do I just not know where to buy coils of copper tubing, or is the bulk discount really that much?

I use 1/4 copper tubing that I had laying around from my ice maker install.  Sure, it's not as neatly coiled as the ones you buy, but that doesn't make it work better.  Just have to be careful you don't bend it too sharply and kink it.

Hard part is adapting it to fit your sink faucet.  For that I slipped a piece of 1/4" ID tube over one end of the copper.  For the faucet I got one of those screw on adapters made for re-routing water through a countertop water filter.  It has a lever to direct water either out the faucet as normal or through a 1/4" hose barb.

What you are not converting is the amount of tubing you are getting vs what you are buying prefabbed.

I had my plumber neighbor install a standard faucet spigot in the cold water line under my sink. No more messing with adapters on the faucet. A small improvement I recommend to everyone.