Calculated on the basis of parts alone, the markup is not large in this niche product area, and I think one should not exclude Mr. March's other costs, such as testing many other DAC boards before he found the Khadas tone board.
The benefit of no potentiometer outweighs the cost, if any, of digital volume control, which has better channel matching and no mechanical parts to fail over time, as potentiometer parts will. In modern playback software, the volume control is usually handled in 32 bits and then, if your DAC requires it, can be dithered to 24 bits. There will be no degradation that human ears can notice.
See here for some observations on the Foobar 2000 volume control. I imagine that Roon's works similarly.
Agreed. The cost of testing DAC boards might even exceed that of just designing one yourself. I look forward to how the amp performs.