I could be wrong here, but here is my reasoning.

At the moment we cannot fly in the old world.
So there isn't a lot of farming going on here.

At the same time, there will be a huge demand for herbs, ore, cloth, etc.
Chances are, most people will just want to level initially, and purchase trade skill stuff down the road.
However, there are also those who want the skills as they level, and there is a limited supply.

During Week 1, Peacebloom/Silverleaf/Briarthorn, Copper/Tin, Linen/Wool and the like will sell very well, because of limited supply and huge demand.
Similarly, Week 2 and onwards, higher mats will sell.

With flying in the old world, farming becomes much easier.
So long-term the price of old-world materials will likely fall from their current pricing.

But Northrend will likely be dead, the same as Outland is now.
Aside from the Worgen/Goblins and new DK's, most of whom will burn through the content quickly to level, and not farm it so much.



I look at Ore vs Bars...

If you want the bars, chances are you either have a miner or can pay one to smelt the ore for you.

And if you're a miner, one of the better leveling methods is smelting.
Chances are jewelcrafters won't smelt much until they reach whatever the last ore type is (Saronite/Titanium equivalent).

But the ore appeals to miners.
And can easily be smelted for those who prefer bars.

Ore has sold very well, in previous expansions.
On my server, the first week of both BC and WotLK, copper and tin ore went for 100g a stack.