Average damage taken for saving throw
Here are some graphs showing the average total damage taken across very many saving throws. In this case, 10,000,000 saves were rolled for each data point.
On each chart, 100% represents a save with no bonus and damage of full on fail and half on success. Each column represents percentage of damage taken for a save at that DC assuming some specific bonus to the save.
Note that these tables assume the DC is the number you need to roll. So a DC 13 save for a PC who has a +3 would show up as a DC 10 below, whereas a DC 16 save would show up as a 13 below (since with a +3 you need to roll a 13 or higher to beat a DC 16)