It is going to cost money to buy phones, unpack them, upload the OS, test the product, then repack them and ship them. I would expect that to cost at least $20, probably more.

Well, proxysto.re does that with GrapheneOS: https://shop.proxysto.re/de/i/pixel/

€50, based on the discount on stock ROM, although €20 out of that goes to GrapheneOS apparently. Based on donation amounts, they sold 67 of them in 2024