It's important to distinguish between things your character would almost assuredly know that you, the player, might not, and things that might be annoying metagame-wise.
An example of the former is "can I jump this chasm?" Is it trivial for me, difficult, risky? That's something any heroic protagonist just knows. And, it's something you, the player, need to know to actually play the game. Not just in a sense of not wanting your treasured PC to plummet to their doom for no reason. But, in the sense that without that kind of knowledge there's no sense of drama or storytelling.
Furthermore, things like that are necessary to distinguish characters from each other. Batman is stealthy as all hell and good at breaking and entering. Superman is not. We need these things reflected in their character sheets. d20 does this in a pretty straightforward way. Batman's stealth skill rolls are so high relative to the average person that he'll virtually never fail (and he has Skill Mastery for added insurance). I can say my character is a "master thief" b/c when I look at his Open Locks skill and compare it to the DCs of locks, I know that he can reliably beat them. I know that my Ranger can track like Aragorn b/c of the rough DCs described in the DMG, or that my Ninja is a master acrobat b/c he can walk a tightrope in the rain while blindfolded.
This is one of the great strengths of the d20 system (WoD, for instance, sort of sucks at it).
@Demelain's Story
In my humble opinion both DM and player were metagaming. Assuming the PC had been hanging around in a desert environment, most likely guarding caravans, for any period of time, knowing the price of a camel is not exactly secret knowledge. It's not like quoting the save DCs of various monster abilities or anything. It's common knowledge in that environment. The DM was metagaming by using a very heavy-handed WBL approach. He could have more subtly corrected it as time went on. Or, he could, I suppose, have argued that they were flooding the market for camels, and therefore driving the prices down, though that'd have to be an awful lot of camels to do it.
Where the player fell down was that he was, I assume, not playing his character. He was playing a murderous psychopath, and a petty one at that. I mean, had he just adventured for a while he would have made a lot more money than a bunch of camels.