Any number is arbitrary, so think about what story goals the money fills:
This is a devil's deal. In these kinds of stories, the money is enough to fundamentally change the fortunes of the family. It doesn't have to be a lottery jackpot, but enough, for instance, to pay for the remaining childrens' colleges, or get a house in a good neighborhood, that sort of thing. And, practically speaking, it has to be enough that the father can pay for these shadowrunners without jeopardizing his plans for the money.
Also in these stories, the people usually squander the money and end up back where they started, because they're bad people for selling their kids.