Well, if you think guns are overpriced because the materials only cost $50-$100, but you're paying $500, I've got a solution for you: build your own guns. Oh wait... It isn't that easy. You need machinery, engineering knowledge, kinesthetics knowledge (so you can design a gun that actually works with the human body), and all sorts of time and effort.
So if you're a firearm manufacturer, you've invested lots of money into machinery. You've invested lots of money into MAINTAINING said machinery. You're paying individuals who have more expertise in their little finger than some of the people here have in their whole body to DESIGN firearms. You're paying people to operate the machinery to produce the guns. You're paying sales representatives to go to gun shops and talk up the guns, so that gun shops will actually carry the gun in stock. You're paying lots of money all around, and only a small portion of that money is actually going toward the materials.
You're also trying to make a profit, because this is the USA, a capitalist nation where profit motive is still the primary motivator for business and the major reason businesses are created and expanded, despite Obama's idiotic rantings about how "you didn't build that."
So, back to the original question: are AR parts and accessories overpriced? Maybe. That depends on the part or accessory, and where you buy it. Some places sell things at high prices. I try not to shop at those places. However, I'd rather pay a little extra for a particular part or accessory and KNOW it works, rather than pay less for an inferior product that may fail, potentially catastrophically, during use.
Scopes are a good example of price vs. quality. You can go buy a Tasco scope for $50 at any Wal-Mart, and put it on your rifle. Good luck, because it isn't going to be a very good investment. You can go buy a nice Leupold for about $250, put it on your rifle, and trust it for years and years.