Best Buy's business model is to overcharge people for peripherals like cables and warranties. Their thinking is that when you are buying a big ticket item like a TV, you compare the prices of the item and not the total costs of all peripherals and additional warranties. So Best Buy says "let's have the big ticket item be the cheap thing that brings people in, but then we will get them on the price of cables." And of course their salesmen are trained not to let anyone exit the store with a TV or a computer without buying a bucketful of cables and extra warranties.

So yeah, that's why their cables are expensive, they are their profit center.

It used to be that Radio Shack was a good cheap place to buy cables, but they also raised their cable prices. Radio Shack could have made a fortune just selling cheaper cables to Best Buy customers, but instead they decided that they could compete with Best Buy and raised their cable prices accordingly. Of course Radio Shack stores are way too small to sell big ticket items, so they failed and Radio Shack filed for bankruptcy.

So now the internet is the only place you can get cheap cables.