I assume that you are referring to example 8 in our free lecture notes. In which case the variable/marginal cost of Y (which is the product that is being transferred) is given as being $70 per unit in the question.
(Have you downloaded the lecture notes, because there is no point in watching the lectures if you do not have the notes in front of you? Also, all of the lectures are indexed in order, and are accessible free of charge, directly from this website.)