Do a bit of quick math - how much did your materials cost? How many hours did you spend on the artwork in question? How much did the frame cost, and how long did it take you to frame it? Think of a reasonable hourly rate, and apply it to your artwork. You can also add or take away from the price based on the work itself - an intricate ink drawing is probably worth more then a simple, framed pencil sketch. And then, since you're an artist and you're allowed to be fickle and tempermental, you can even jack up or lower the price depending on what you think of the customer!