High end electronics (for the most part) and diamonds have incredibly high mark up. The thing with diamonds is that Debeers own something like 85 or 90 % of the world's diamonds market share and they can pretty much dictate the price to everyone, and why would the other 10 or 15% argue with them when they can make the same insane amount of profit.
With plasma TV's you're paying for the technology and the eliticism of it. As time goes on, plasma's should come down in price, especially if some new technology hits the market.
Now if you want to talk about insane markup, lets talk about the soft drink industry. A 10 oz soda in a cup that you pay $2 for at the bar doesn't even cost them a whole penny. Soda you get in a plastic bottle or can costs a little more to make because of packaging, and it also has to pass through the hands of extra distributers.
Fact is, the price will be what the market will bear.
Now if you find a wholesale ring that is half the retail price, I doubt there is much room to negotiate, but that doesn't mean you can't try.