While in the TED talk Singer may give the impression that he endorses the claim that one can save a life by donating $200 to a cost-effective charity, his actual position is more cautious, and less vulnerable to Jonah's objection. In The Life You Can Save (p. 103), he writes:

It’s difficult to calculate how much it costs to save or transform the life of someone who is extremely poor. We need to put more resources into evaluating the effectiveness of various programs. Nevertheless, we have seen that much of the work done by charities is highly cost-effective, and we can reasonably believe that the cost of saving a life through one of these charities is somewhere between $200 and $2,000.

For context, see this comment.