Well "shore power" can be seen as ground power and that is comonly used and airlines pay for it, so billing in some way is already there. However, it is quite often very expensive so that the APU is cheaper. Add to that that you need both ground power and external AC commected to replace an APU in anything warmer than 20 degrees C or colder than 10 and it becomes complicated. The 787 is the only excpetion as it only need electricity, just lots of it.