Everything is negotiable. I would have figured the miles to the next busy express center, added them to the offered load miles and come up with an amount I would do the run for. (All miles) Then the ball is in there court and they can decide if you covering the run is worth paying you what you think is fair for you to do that run.
Outstanding reply! It is respectful of yourself and your carrier and it brings any game playing to a quick end. It is an excellent way to run your business but also has a down side.
You must be prepared for the possibility that your carrier will decide that it does not want self-respecting owner-operators in its ranks and will ditch you or let you sit and rot in favor of one who can be more easily manipulated and gamed.