Right now, there's a trend--especially at the high end of raiding and PvP--of characters taking two crafting professions. They do this in order to get the "perks" from each profession slot, maximizing their character's potential.
This has some interesting economic ramifications. There are now fewer gatherers, and more crafters, so the raw materials have become scarcer, and prices have increased significantly. To combat this, Blizzard is introducing perks for the gathering professions in WotLK. For example, Mining gets a +Stamina bonus.
However, some people aren't happy with this, because levelling crafting professions is an expense, while levelling gathering professions gains you money. They feel that because they put in the greater expense, they should get a greater reward.
The problem with this is that money or gold is really irrelevant when it comes to character balance. If Leatherworking gives more of a boost to your characters stats than Mining, Mining will be dropped by the serious players, regardless of how much money it brings in.
It's like there are two mutually exclusive choices. A crafting profession can either be useful to your character (I.e. it has benefits that cannot be provided by another crafter), or it can be profitable. If it is useful, everyone will take it, increasing the supply, increasing the costs, and reducing the profits. If it is not useful, then fewer people will take it, increasing scarcity and making it more likely you can make money with it.
For example, if Blizzard really wanted players to be able to sell crafted goods for profit, they should increase the perks for the gathering professions until they are noticeably better than the perks from crafting. This means that most serious players will drop crafting professions and go double gatherer. This means that there will be plentiful supply of raw materials. Then Blizzard should have crafted BoE recipes of good quality available. Plenty of raw materials + fewer crafters = lots of income.
But would you take that trade? More gold in exchange for lower character prowess? I think that most people won't. That it will just lead to complaining by everyone. So Blizzard will try and balance perks between the crafting and gathering professions.
Unless the cost is extremely exorbitant, gold costs never hold players back. Respec costs haven't, costs to level professions haven't, repair costs haven't, alchemy costs back in WoW 1.0 didn't. You cannot count on gold costs to balance character prowess. All significant gold costs really do is cause players to spend more time farming, complaining, and cause players to drop out of the game when they can't keep up.
Edit: This post is really the confluence of two somewhat-related ideas: gold costs do not affect game balance; and crafting can be useful or profitable, but not both. I probably should have tried to separate them out a bit better.