Bob:
If by cost/benefit analysis you mean what risk such an act entails for your life or the life of the ones you love, I agree with you.
But once you have established that your life runs negligible or no risk, what other sort of cost/benefit analysis could you possibly make?
Such an analysis would require information not only related to what you know you will lose (e.g. your job interview) but about the stranger and the consequences of saving his life. Is he a millionaire who will pay you back with a big check, or an Islamic terrorist that will interpret your act as an act of Alah that kept him alive to fulfill His will to blow up some bus?
Without information of both risks and benefits, how can you run a risk analysis?
That’s why I would say: In those situations, trust your nature.
Evolution has done a great job in you.
It has given you a mind able to perform logical thinking, but it has also given you neurohormonal mechanisms to make you do the BEST possible thing, in terms of survival, when there is no time or information for a logical thinking.
You can decide in advance that, in situations where rationality will have no time or input to work, you will trust your nature.
And many times it will guide you to an adventageous position. As a tool of survival, mind trumps guts by far, but guts trump chance. Following your guts is better than flipping a coin. I would even go further: whereas mind trumps guts by far in medium and long term goals, guts trump mind in immediate term goals, when decisions on life and death have to be made in few seconds. Indeed, trying to apply logical thinking such situations would work against you.
I believe that it is in our nature to save strangers when doing that does not represent an ostensible risk. Maybe evolution made individuals who save strangers more able to be saved by others when they are the strangers. This behaviour is seen in other primates. Sympathy generally pays off.