There is no reason to financially penalize people who want soy milk instead of cow's milk in their coffee drinks. Many people are vegan, prefer soy milk to cow's milk, or they suffer from lactose intolerance. Replacing one beverage add-in with an equally priced substitution should not end in a higher priced final product.
Do not discriminate against vegans, people with dairy allergies, or people who dislike cow's milk.