The foil cost to retailers is of coz more than a regular book. And you are right, we are basically only charging the extra to cover up the printing of the foils, the extra artwork, etc.
Unlike the regular book, which is offered to retailers under the standard Diamond discount system (that is depending on the quantity of books ordered that month, retailers will have a discount between 50% to 35% off cover price - the more they order, the higher their discount is), the Power Foil is charged to retailers at a fixed NET cost. Meaning a fix price. However, these incentives normally do not have a cover price set. So it is up to the retailers to decide, probably based on the demands in that area.
Hope that answers your question.