Cecil...no additional costs at all if you are already pumping your water and aerating....just simply redirecting the costs expended.
Your system already beats out all harmful gasses and adds as much oxygen as it possibly can to the well water. The only "loss" is the extremely minor bit of pond water being aerated at the dump point. Dumping your water in the top of the pond and letting the "gravity" work to pull the more dense cool water to your pond bottom is also letting that cool water be warmed far more than needed. Adding a more confined, insulated space for the "agitation aeration" system you have is going to save you energy.
Extending your tubes to the bottom would save you considerable energy because the water would stay cooler, longer and less water would need to be pumped.
