Not that I really know, but if one takes into account the total cost of running BPL (i.e. salaries, rent, equipment, financing, etc.) -- and not just the difference between online updates versus printing and paper -- then maybe the difference in total overall cost isn't all that big???
Another way of looking at it -- perhaps most of the cost is in putting the information together -- and therein lies the value -- versus the particular mode of delivery per se...