Are upgraded power cables really worth the money?

My system consists of musical fidelity a5 integraded and cd player, b&w 703 and a rel 305 sub. I'm Im using purist audio musaeus interconects and speaker cables. I understand and can certainly see how these cables have helped my system, but am a bit aprehensive about investing in upgraded mains cables. Do they really make much difference? With the system I have what would be the right amount to spend on them. Any suggestions?