Rebbi Ami doesn't solely mean that the last one given has to be worth a Perutah. He just means that the last one (or ones) left when he finishes stating, "with this one, and this one, etc." is worth a Perutah. True, the Mishnah seemingly means that she is eating each one he is giving her. However, this is not necessarily the case, just as the Mishnah is not limited to three dates because it only said, "b'Zu" three times. Accordingly, Rebbi Ami means to explains that the Mishnah means that as long as there is one (or a few) left at the end of his statement, it is a valid Kidushin. Rebbi Ami said, "the last one," because that is the simple meaning of the Mishnah. However, it is clear the Mishnah is not necessarily limited to the last one given, but rather the last figs remaining uneaten. [See the Pirush ha'Mishnayos from the Rambam on this Mishnah. It seems to me that he is touching upon this question, and giving a similar explanation.]

All the best,

Yaakov Montrose