1. Look at the mats required for the enchant on this site.
2. Also look and see if the enchant is a world drop, faction reputation or a trainer one.
2. Go to your local AH and check the price for mats, and the world drop formula price. If the formula isn't there, you can be assured that it's a very rare and pricey.
3. The enchant price would around the AH mat prices for trainer recipe, and a larger tip for world drop or faction reputation ones.