$0.50 to get it manufactured somewhere in Asia, $4.00 to get it shipped to the US, tested, packaged, put on a shelf, retrieved for your order, and put in a box to ship your way.
You're not paying for the PCB, you are paying for an American warehouse worker to do their job without having to pee in a bottle and walk around dead coworkers.
Yeah, I once had similar (with a different purpose, but very alike) boards of my own design produced and paid $22 total for 5 assembled boards, including shipping from China to Europe and all taxes/customs. Unlike this one it had SuperSpeed passthrough too, so you could probably go cheaper :)