I give my boys 6 cups each once a day and they still want more.
I know you say you can't afford food better than Purina, but consider this:
I feed my 70lb dog TWO CUPS a day of Wellness. One bag lasts me for two months.
You are feeding 3 times what I am, so how do you figure it's cheaper? For every bag of Wellness I buy, you buy 3 bags of Purina. Definitely not cheaper.