There doesn't appear to be any set method, but expect to give a lot away for free at early levels. The trade off is at least you are leveling your enchanting skill. When you start doing it solely for profit, it's up to you. Personally, if I provide the mats I charge what I paid plus around 10% for myself. So if I spend 15g on mats I charge around 16.5 or 17g. If they provide the mats they generally will tip but that's up to the buyer's discretion. If they don't tip don't do enchants for them anymore. The best thing you can do is establish reliable customers. When you sell enchants in town, especially if you're doing them for free, ask them to add you to their friends list and use you exclusively for enchanting from that point forward. I get several tells every night from customers who keep coming back to me. This also makes pricing easier since they know how much it's going to cost from looking at the mat prices at the AH.