Somebody on an internal alias asked, "why is a HDMI cable $60 when the DVD player is also only $60", and here's what I wrote:
McDonalds sells you a hamburger for $0.69 (hypothetical price - I haven't bought a burger from them in quite a while). They charge you $0.89 for the coke that goes with it. The margin on the burner is miniscule (the cheap burger may even be a loss leader). The margin on the coke is about $0.80. People are price-sensitive to the burger price, but they're not price-sensitive to the price of the drinks.
Same on A/V components and cables. People are price sensitive on the components, so most companies keep their margins there very low. But once you've decided to buy it, you need a cable. You can a) try to go to another store to get a decent cable at a decent price (but none of them sell that) or b) buy something over the net and put off your gratification until it shows up.
Most people do a, and the people that do b usually buy the component that way as well.
So, that's where the stores make their money. It would not be uncommon for the store to make more on the $100 you spend on cables than the $1000 you spend on components. And it's a lot easier to tout the "you can spend $60 on the normal cable, but you will probably want the oxygen-free copper one to get a really good picture" line than it is to push extended warranties, which require you to talk trash about the very component that you just talked up.