It's nonsense that these cables are so expensive. Nintendo put a digital-analog converter chip inside the cable and basically decided to never release the design so no third parties were ever able to manufacture compatible cables.
My recommendation is that you might want to try Google first to see if somebody has successfully reversed engineered these component cables before spending $300 for originals. With today's technology, I honestly don't believe that it would be that difficult to backwards engineer the cables even if Nintendo doesn't release the chip schematic. Plus with the insane price that these cables are being sold for, I'm sure that the demand is definitely there for somebody to invest some time to actually create a usable alternative.
I can't confirm this, but apparently the GCN component cables actually produce a better image then the Wii ones do. I'm sure that any difference is fairly small, but I think this is likely one reason why the GCN component cables are so popular.