When SY was asked to look into Madeleine's disappearance there was, as far as we know, no live leads and a reported budget of 5 million. £800000 has been earmarked for, Kerry says, substantive leads. Surely even you can see the disparity in those figures ?
I would think the process was quite different, unless British police use weird project management techniques that other projects do not.
I would expect the SY review (and the Ben review) to sift through the available information, to determine first whether any viable approaches existed. Then it would scope the project and estimate the manpower and costs required.
At that point, someone or more likely several people further up management would look at the proposed project, the estimates, the likelihood of success, and decide to press the go or no-go button.
It is not as simple as a cost per child calculation. I don't know the Ben case, so if anyone wants to summarise the complexity, please feel free to enlighten me.