The general recommendation is to spend 10% the cost of the entire system on cables. This would include the speaker cable, interconnect, power cord, USB, digital and other types of cable.
Personally I have spent about 25% the cost of my entire system on cables. In my experience, speaker cables and interconnects are equally important. Power cords, not so much. I will be upgrading the USB cable very soon and will see if the difference is worthwhile for the price paid.
