I was attracted to this because it's similar to a Montessori program (which would cost more to get the materials for) and my son's been in Montessori for two years and is very attracted to the math materials.
I think you need all the materials if you want to do it all by-the-book, which I think I would want to do, especially since this is my first year. I'm not sure that savings would be that great to DIY-it or get elsewhere when you consider the time involved in doing that.