In this example I wouldn't hold someone morally responsible for the murders if they failed to pay $100 ransom - that responsibility still lies firmly with the person taking the hostages. Depending on the circumstances I would probably consider it morally questionable to fail to pay such a low cost for such a high benefit to others though. That's a little different to the question of moral responsibility for the deaths however.

Note that I also don't consider an example like this morally equivalent to not donating $100 to a charity that is expected to save 10 lives as a utilitarian/consequentialist view of morality would tend to hold.