USB Cable prices are not necessarily indicative of water cooling supply prices. They fluctuate from site to site, and they are definitely part/component dependent. That's why I ended up purchasing parts from so many different places.
Jab-tech has below average (i.e. better) prices on most supplies than places like FrozenCPU, though svc.com had the X2O 750 for the same price. Their fans are also really cheap for some of the good ones (Yate loons, Scythe Slipstreams), but they don't have a ton.
FrozenCPU seems to have the best selection of almost all parts, and they had the best waterblock prices I found (both GPU and CPU). I just happened to get good prices on used blocks.
Petra's Tech Shop has a lot of the little accessories that you will most likely need (additives, fan grills/filters, vibration reducers), but not so much the major hardware (at least when I ordered last week).