I bought a button making machine, and needed a couple of circle cutters for cutting the art - but they are like $5, and I was like "whaaat?"

Very simple - three parts. Tap both the holes in the blade holder to accept a #6 screw each. Take a normal snap-off type blade, snap off two segments, and insert them into the slot in blade holder, and use one screw to secure it in place. Then use another screw (you may need a washer also), to secure the blade holder to the arm (adjust to the size hole you need). Then slip the arm into the foot, and presto.