If you want actually decent performance, you need a board with real I/O available (like PCI-E and SATA). And full speed gigabit, if you want to be able to access your data at reasonable speeds.

And if you want it to be quiet + cool your drives, that costs money. Not to mention the software — Is this bare metal or does it come with a base OS? Does the vendor offer driver support or are you stuck with ancient Debian?

What about manufacturing volume, is it 1000 units or 10 million units?

Those all will influence the price, especially for something fairly niche. ;)