Just the opinion of an old farm boy,but......if it were cheaper or easier farmers would never buy feed from a mill/feed store. As pointed out above,the correct ratios,digestability,protein,fat and nutrient content is a science unto itself.  Of course farmers are making a living with their livestock and have to have the best feed for cost vs production. Backyard enthusiasts can be less concerned with that equation but you still want the best you can afford.
I realise this topic is directed toward the "organic" crowd and i won't get into that except to say that route will never be "cost effective". You go there strictly because of personal convictions. What we do,and seems to work fairly well is to grow our own grain crops(millet,sunflower,sorghum,etc)to blend and feed in conjunction to purchased feed as an "extender". We live on 3/4 acre and do this alongside our veggie garden so it's not a big operation bit it helps on expenses.
Larry