We're just dealing with two different definitions of success here aren't we?
Success Definition One: Success occurs when the child is returned. This is a binary outcome and you're right to say that if the child is returned then it simply is a success under this definition and nothing can change that.
Success Definition Two: The scheme works and is practical. This is a qualitative outcome which does require comparing apples to oranges (or as you emotively put it; "putting a price on a child's life"). If Bill Gates offered 12 billion dollars for the return of a child, no questions asked, no law enforcement involved, it might work very well indeed, but cannot be called a success under this definition.
With regard to the 'prior condition', I would still argue that if the scenario under Two above occurred, and worked, I would still call it a failure under definition two and a success under definition one.
And honestly, definition two is the more interesting one to try to produce a solution for.
Success Definition One: Success occurs when the child is returned. This is a binary outcome and you're right to say that if the child is returned then it simply is a success under this definition and nothing can change that.
Success Definition Two: The scheme works and is practical. This is a qualitative outcome which does require comparing apples to oranges (or as you emotively put it; "putting a price on a child's life"). If Bill Gates offered 12 billion dollars for the return of a child, no questions asked, no law enforcement involved, it might work very well indeed, but cannot be called a success under this definition.
With regard to the 'prior condition', I would still argue that if the scenario under Two above occurred, and worked, I would still call it a failure under definition two and a success under definition one.
And honestly, definition two is the more interesting one to try to produce a solution for.