Complement it with the fact that it costs about 800 thousand dollars to raise a mind, and an adult mind might be able to create value at rates high enough to continue existing.

An adult, yes. But what about the elderly? Of course this is an argument for preventing the problems of old age.

that is a good argument against children.

Is it? It just says that you should value adults over children, not that you should value children over no children. To get one of these valuable adult minds you have to start with something.