cable upgrades , real or rip off

im sure this has been done to death on other threads, but im somewhat new to the high end audio thing. but i must ask do these items make a huge diff. , a small one or something in between. i cant help but asking myself why when a company is trying to produce a knockout item such as a intgrated amp that costs say 3000.00 would they skimp on something like a 300.00 powercord if it would make there product sound that much better?