The prices seem fairly in line with what you normally see for fully open source boards. They cost a lot because of the low volume and huge amounts of work that have to be done. For a regular board you just slap down whatever parts you want but if it has to be open source you need to put in a lot more work looking for parts with free firmware and spec sheets as well as writing a lot of firmware yourself.

The GnuBee is an open source nas that is actually cheaper than everything else on the market. I suspect because its a fairly simple device and the proprietary versions are super marked up as well as the creator being located inside Shenzhen