By "save" I meant "avoid losing" not "gain an extra." Assuming a child would not normally get mercury poisoning, for example, by preventing mercury exposure I am preventing my child from losing some amount of cognitive ability.

My guess is that interventions like preschool are more likely to fade with time, and brain damage is less likely to fade.

I'm not taking Salkever's numbers literally. But you probably agree that brain damage causes lost value, possibly a lot of lost value. I estimate that I may spend a few thousand dollars on various steps to prevent brain damage to my children. That seems like a good investment to me.