If you don't have it, there's an addon called ReagentCost that is awesome in helping to determine what to level on. It tells you the costs of making an item based on AH prices on your server(I have it paired with Auctioneer but I'm semi-sure you can use another AH-pricing addon). Saves a lot of gold if you're planning on buying your mats. Otherwise, just do as was suggested by others, because farming yourself will always cost you the least amount of gold outright(gold lost due to not selling what is gathered makes it murky, but meh).
On another note, pairing ReagentCost with Auctioneer lets me decide what will sell for more than it costs to make...if it will actually sell, that part takes some trial and error.
My final suggestion is to take an alt to level 5 and add a third addon, Enchantrix, to the formula. That will tell you what an item disenchants into as well as the average value of those enchanting mats(even when you're looking at the item in the crafting screen), so you can make things that disenchant for more than they cost if you're lucky and enchanting mats have no AH listing price, so good times(plus I've never had mats not sell after a few tries listing).
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An awesome fan-cover wrote: