I've done several sets of curtains and drapes. I always work by the hour on something like that and try to give them a fair estimate up front, like "I believe it will take me 4 to 6 hours at $## an hour." Then I carefully track my time and work as efficiently as possible. With the price of custom curtains and draperies, paying to alter ones you already have can be a real bargain if your windows are non-standard sizes. (The $## varies depending on whether I'm doing a favor for a friend, or whether it is purely business.)