the cost per seat mile argument has its merits when discussing frequency on a route. However, if the airplane isn't full CASM doesnt matter. An RJ costs less than any mainline airplane to fly on any route. they are smaller airplanes that in turn burn less fuel and have lower maintenance, insurance and other variable costs (landing fees etc). Now a A320 can divide more of those costs out to a greater number of people but if there arent enough people on the airplane you will lose money. An RJ is designed to make money with smaller loads. The full A320 might bring in more money thus a higher profit margin than a full Crj, but a half full CRJ is going to break even where as a half full airbus is going to be bleeding money. Of course none of this means sh!t if you arent charging enough for the ticket to cover the costs when the airplane is full any ways.