Digital cables are probably the items where a small increment in improvement will cost the most money.
The Audioquest Eagle Eye digital with the dBS (battery) system is obscenely expensive in my view, but not a bad purchase if you can find it used.
As good as it is with its price of around $ 750.00, it is only marginally better than another cable I paid $ 250.00 for. But it IS a bit better and you CAN hear the difference in the extra resolution and transparency. But I could live with the cheaper cable of course.
Whether it is a worthy investment is of course highly debatable.
I used a very non-scientific approach to justification: Given the substantial price of my speakers, pretty fancy integrated amp, CD player and Dac, I figured it cost me $ 120.00 per component to upgrade them all !
Had I discovered this cable before, it would have prevented me from selling my previous amplifier for sure.