I am so sorry you are in this bind. I have become very careful in responding to requests from others. Most non-quilters/sewers have no idea of the the cost in time or materials to make even a simple project.
I would not make her a table runner for $12. It just cannot be done...your time alone is worth more than that! If she insists on knowing how much it will cost, be sure to allow enough for the fabrics, and your time in selecting them and your time for making the runner. Chances are, the price you come up with will make her feel faint and end the matter.
AND REMEMBER, fabric is expensive and your time is worth something. Don't allow yourself to be taken advantage of by her or anyone. Be honest with her about the "real costs" and see where it goes. You do not owe her an apology if it is too costly for her.