What I've done is:
1) Pick a minimum price for raw enchanting mats. Say, 6g per dust, 20g per larger essence, 15g per dream shard, 100g per abyssal. These should be prices that raw mats in the AH stay comfortably above (if not, you have a problem).
2) Assume green armor DEs to 2 ID, green weapons to 1 GCE, blues to one DS, and purples to one AC.
3) If the price of the materials is less than the output, buy it.
Where this strategy has fallen down is in leveling enchanting, I still need to do actual enchants and so am listing vellums on the AH, which are hard to find a profitable price point for, because of the perceived cost/value.
For example, I listed
[Scroll of Enchant Boots - Greater Spirit] for 200g. My cost was, at most, 182g (with a 10g vellum), so a nice, low, profit. The cost of raw mats on my server is somewhere around 300g. I get interest when I announce that it's available, but it still sits unsold, and has been listed twice. People say things like "I can get mats for WAY cheaper than that" but I'm not exactly sure how they are doing it, if they are not an enchanter.
Once I get to 450 enchanting, I'm definitely using this strategy to unload mats only.