I don't charge a fee very often as I mostly offer enchants to guildies (tip free then of course), but generally speaking I'd ask for 100g if I was enchanting one of the top end recipes that not everyone has, which is a small amount compared to the overall mat cost and I've never had any complaints.
I usually don't charge fees but these 6 enchants took quie a bit of gold to get, so I wasn't sure how to go about regaining some of my investment back.
I dont do enchants in person, its 10x the work for 10% of the money. I sell on the AH exclusively, with that said, market price of mats plus 15%, that works out to several hundred gold for maelstrom enchants. I also try to buy maelstroms at lower than market prices (and pocket the difference as profit) since I have the benefit of shopping the market at all times rather than the average player who wins a loot roll and is immedietly subject to whatever the current crazyness might be on the AH.
I dont sell a ton of epic enchants, but I also dont work hard for it, so its a tradeoff I am willing to make.
of course with 4.1 coming Ive stopped making enchants, I dont want to be sitting on a full line of these enchants when the value tanks.