It is a theoretically good theory, but not a good idea. In general, doctors recommend folks not do this. [1] A few sites I visited warned folks not to tell their doctors they were feeding the baby homemade formula. One site even said I needed to watch a bunch of additional things, such as whether or not the baby is urinating properly and enough [2]
In addition, not everyone lives near those sorts of stores - when I lived in the states, I would have had to drive an hour or two to get to such places and I'm not certain the ingredients are readily available there. Furthermore, there is the issue of time. I'm pretty sure workplaces in the states must allow employees time to express breast milk, but do not allow time to make formula. Recipes range from all day to hours at a time: I'm unsure of the spoilage times on these.
Furthermore, not one of the recipes came from a site I would trust at first glance. I don't trust myself to know the nutritional needs of children enough to do this, let alone a lot of other parents uneducated in this area.
Also, I don't have children. I'd personally only buy this as a gift to help a friend, and only after asking first.
Most parents probably couldn't even manage to parse the research literature to develop a formula of comparable quality to the commercial ones on their own (the published recipes I've seen on the internet certainly haven't even tried), much less source or prepare the necessary ingredients.
And if you haven't researched, developed, tested, and manufactured an adequate supply of your homemade formula before your kid is born, you probably aren't going to want to do it afterwards, even if you have the required skills.
