One of the basic "rules" I started with when I started playing World of Warcraft and had no other information to go by was this:
1) Determine what prices to auction something in the Auction House/Player Market according to the quality that Blizzard has assigned to that item.
Meaning, if a piece of armor was Blue in color (denoting "Rare"), I would list it for higher than something that was Green in color (denoting "Uncommon", a step below Rare), which in turn would be auctioned for higher than something that was White in color (denoting "Common", a step below Green).
I now have one thing to say about this initial assumption: In terms of battle equipment - weapons and armor, the assumption is a fine one to stick with. In terms of non-battle equipment, the assumption is completely Wrong!
White (professions) Items Value Skew
One of the first items I fished up that challenged this perception was that of the recipe for the blacksmith's Steel Weapon Chain
Trade Goods
Requires Blacksmithing (190)
Use: Attaches a chain to your weapon, making it impossible to disarm.
Charges: 1
Item display is courtesy world of warcraft.allakhazam.com. ',VAUTO,HAUTO,FULLHTML);" onmouseout="return nd();">Steel Weapon Chain. I got it while fishing in Stranglethorn Vale the first time, and since I didn't have a Blacksmith at the time, I sold it for 1g in the Auction House and was quite pleased with myself...
Until I went to Thottbot and looked at the notes about this particular recipe and how rare it was, how it only dropped from chests and fishing boxes in a very small area of the game, and how players were willing to pay up to 20g for the pattern!!!
And, this pattern was a regular "white" pattern. I thought that if the pattern were rare, it would be a different color, but I guess that's not the case!
Valuation of Professions Materials
There's no easy way to tell which profession materials or which professions recipes are worth more or less than one another except to look up each item on Thottbot or a similar site with player notes appended, and see what everyone has to say about it.
Alternatively, there are addons that can help determine value, like WoWEcon (which has a downloadable database) and Auctioneer (you have to build the database by scanning the AH at least once a day over a few weeks).
Keep an eye out for things you might not otherwise think are valuable, because every bit of value is a small niche market waiting for you to provide and gain wealth from!