the difference in cost of USB converters is mostly due to four factors:
1) the inclusion of a low-jitter clock, such as Superclock4.
2) external power supply
3) custom software driver (not using the stock Windows driver)
4) optimum design and parts selection, regardless of cost
These four factors drive the cost up orders of magnitude, but also result in sound quality that matches or beats the best transports. It's like the difference between a Samsung $60 DVD player as a transport or an Esoteric.
Yah Steve. Agree with you. There's always a high cost if we want to squeeze the last drop of performance out of something. Out of the 4 points you listed, is No.1 the most costly?
As you said USB is merely a method to get data out of a computer, yet I think it's one of the greatest technology invented, for us computer audio freak. :D