HDMI cables
A long time ago in a mall far away, I used to work in computer retail. One of the most profitable items we sold was printer cables. A parallel cable that sold for $79 would cost the store under $10. This dwarfed the profit on the printer.
You'd think we as a consumer electronics buying public would have figured out this scam by now, but no. Bought an HDTV lately? Been able to leave the store without someone trying to sell you cables?
Monster has built a business around expensive cables. They're the most obvious example, but really, go into Best Buy or Circuit City or Future Shop and have a look at the cables they sell. There's no bin of $3 cables at these stores; they're all wrapped in shiny clamshells and cost a lot of clams.
For analog cables I can understand spending more for better cables; there is going to be degradation if you're using cheap connectors and poorly shielded cables.
But HDMI is a digital signal; it either arrives perfect, or it doesn't. If it's not arriving perfectly, then you'll see it - it's not going to be an imperceptible increase in quality; it's going to show up as big ugly blocks in your picture.
My TV supports HDMI so my coping mechanism for my broken XBox 360 was to buy a 360 Arcade, which comes with HDMI. When I get my repaired console back, I'm going to sell it. The difference between the two prices is my "Upgrade to HDMI" fee.
But the 360 Arcade doesn't come with an HDMI cable. I went to Wal-Mart looking for one (they don't have much tech stuff but it's nice having a solid return policy on whatever you do buy there), and they had a deal on a 6' cable, $29. But they were sold out. I walked next door to Future Shop.
Literally the first HDMI cable I ran across in the store was this one. $188 for a 6 foot HDMI cable. The cheapest cable they sell is $49.
Meanwhile, check out eBay. Without the need to add profit to the sale of something else, eBay items tend to sell for closer to their real value. Searching for an HDMI cable located in Canada finds a few of the expensive clamshells, but also finds an average HDMI cable price of around $8. If you're willing to buy from the states, the price is closer to $4. Plus shipping, of course, but shop around and you can find cheap shipping.
Frankly I didn't expect Wal-Mart to play this game; I expected them to be selling a 6' HDMI cable for around $10.
One of the most useful services eBay offers is being able to give you an indication of what an item is worth (as opposed to what it's selling for). Scarce items like newly released game consoles are worth more than their retail price, and a commodity item like an HDMI cable is worth less.
(At least more printers come with USB cables these days; but you can still pay too much for gold plated super duper USB cables).