I'm considering the knights pay as the estate, land, and titles that he is given by you as his lord to serve as your vassal. I figure that'd go into support cost. And the knights would have their "entourage" with them. Stable boys, pages, squires, blacksmith etc would all be factors in a unit of knights. I can't see a unit of peasants having any expenses but food. Hell, they're probably the ones that carry everyone else's food.