Thought I'd weigh in here a little.
I was comparing a pair of USB cables over the weekend with my Bryston BDA-2 / Bryston BHA-1 / LCD-3 rig - a random cable that came with my Dell monitor and the Wireworld Starlight.
I must have swapped it out at least 10 times to try and isolate a clearly audible difference and truth be told, I was struggling. Now, I'm not saying there's no difference as I'm not sure as to whether I heard something or whether it was placebo but one thing was for sure - I couldn't conclusively hear the difference.
Interestingly enough, after this comparison, I moved on to compare XLR interconnects. I compared a pair of cheap Audio GD XLR interconnects and Chord Company Cadenza XLR interconnects. Without the least bit of exaggeration, I stopped after half a minute of listening because there was no doubt in my mind about the audible difference in favour of the Cadenza's.
After this experience, I'm left with the following thoughts,
1.Perhaps digital cables don't have the same kind of difference analogue cables do
2.Perhaps digital cables under one or two hundred dollars won't provide an audible difference and you'll have to go alot higher to hear something (if it even exists at all that is)
3.Perhaps whether or not a USB cable will be audible will depend on the level of jitter in your source and the quality of USB implementation in your DAC
4.I'd much rather focus my budget on analogue cables (assuming the rest of my rig is downpat) before digital cables