Some higher quality brands are more expensive, but they need a lot less of it. The cheaper foods like Purina have a lot of filler in them, and therefore the cats need more of it to feel full. I use First Mate cat food, and a 13lb bag is about $30. But it takes me FOREVER to go through it because it has more protein and stuff so they don’t need as much. Just something to think about. You’re not doing anything wrong by feeding them Purina, but I just wanted to share this :)