I think that the reason why manufacturers only include short cords has more to do with liability than the cost of the wire. Lets face it, including a 15 foot headphone cable as opposed to a four foot cable would only add on a few dollars at the most to the cost. However, someone tripping on a 15 foot cable and suing the company can cost thousands, even if the company wins the case. Alas, that's why we don't have sufficiently long cables.
That's also why places like Monoprice will always be in business.