Well some enchanters look at the cost of mats in the AH and then apply some sort of percentage as their fee, and if it works for ya, tear at it, i think thats silly, since every other trade in the game it is cheaper to buy the product than the mats, why is enchanting diffrent, dont know.
The irony to this is that the folks with the money and the need for high priced enchants, usualy belong to a nice sized guild, that already has a enchanter that dose it either free or at cost. Their is still piles of money to be made enchanting for "the little guy", just got to advertise.
The awnser to what you ask is that their is no awnser, it all depends on your servers age, the rarity of the enchant, cost of mats on your server, just listen to what others are selling their enchants for, and either match or undercut them.