A few people have said the same thing, and it's true, but it's a tautology; it's true by definition. The real question is, how does your internal price-justifier work?
Monte Cook had a great way to talk about it. The difference is between a gaming product, and game materials. A game product is what you buy --- the book, the boxed set, etc. Game material is what you actually use.
For instance, let's say you buy "Song and Silence". That's the product. Call it P. You select "Quicker than the Eye" for your PC. That's the gaming material. Call it G. The price of the product is $.
My formula for computing value is: P/G TIMES $.
In other words, two questions have to be answered: what percentage of the product constitutes actual gaming material? If a product is 100% material (for instance, you run a campaign based on Return to the Temple of Elemental Evil), then all of the product is used, hence a higher price can be justified.
If in the case of a splat book, you use one or two feats and a prestige class, then it's only say, 5% of the book, then the book has to be of a lower price to justify the use.
Now, there's an additional factor here, which is how frequently you use that gaming material. For instance, "Quicker than the Eye" gets used every game session, if that's your PC's feat. But once you're done with the Temple of Elemental Evil, you might as well throw away the book, because you're done, so some of the value is lost, since it is one use.
The Forgotten Realms Campaign Setting has high value to me at $20, because the feats from the book, and some of the rules are in use in nearly every game, and if you play in the campaign setting, it's even higher value potentially. To me, the splatbooks have low value, because much of the prestige classes and feats and new spells are pretty useless, and there's no potential for me to make more use of them in the game. The Core Books have the highest value to me, since they come into play nearly every game session.