From Georges Duby’s Battle of Bouvines (pg.65). He doesn’t indicate the exact citation – I’m assuming it’s from the Marechale poem – for the quote and since I only have the English edition it’s too cumbersome to phrase search online.
William the Marshal of England, a hero of knightly wars celebrated with an epic song written shorty after Bouvines, one day asked Philip Augustus, with regard to the Poitevins whose about-faces benefited Capetian efforts against the Plantagenet king, why traitors of the sort which in France used to be burned, broken, and dragged by four horses, were now lords and masters. The King answered him: “It’s just buying and selling. They are as disposable as butt-wipes”