What makes a DAC so expensive?

Value for money is a minefield.

Perhaps you can price out the components that go into a unit, but then you have to factor in all the other costs, past, present, and future that the manufacturer has to account for.  I've never seen that done convincingly, because buyers forget items or underestimate costs.

Once you've got over that hurdle, a unit that genuinely costs more to produce and market may not sound as good, at least to some (or many) people, as a unit costing less.

So there's no ultimate correlation between performance (sound) and MRP, however nice it would be if there were one.

So you pays your money and takes your choice.

Look at photos of the insides of the four Denafrips models.  There are very distinct and noticeable differences.

Rubbish!  Brightness is first and foremost a function of the engineering of the recording.  To some extent the venue and other "external" factors, but primarily the ADC used by the recording engineer, plus any post-recording processing.  The early DDD recordings made by Deutsche Grammophon (DGG) were often excruciatingly bright.