One of their kits or a custom loop? Their kits normally come with enough to cool a cpu but are made to be able to expand. A custom loop depending on what your trying to cool, cpu and gpu, can run from 300 - 700 or more. The more parts you want to cool the more its going to cost.
ether way they are better then air cooling or all in one units.
I did a complete custom EK loop that cools my CPU and GPU for only about $350. I have an Alphacool res, primochill tubing, and mayhems coolant, but everything else is EK.