Just a couple of issues with grinding one's own corn.  I don't have a stout grain mill and grinding enough for 40 birds would wear out a light weight, kitchen model too quickly. The second thing would be the corn itself.  Much of the corn sold cheaply for deer feed is not suitable for chickens.  There's some reason that escapes me presently, but a quick search would uncover it.
 
  I pay less than $20 for 100 pounds of complete layer, so $13 a 50 lb for whole/cracked corn wouldn't help me much, YMMV.
		
		
	 
@ Fred's Hens: I agree that commercial grinding operations can cost-effectively mill corn kernels for resale. I am grinding free field corn in my KitchenAid H.D. food processor. I don't buy cheap appliances, whether it's for sausage making, mixing cookie dough or processing food. I'm just experimenting with this idea because the corn cobs (I did have to manually remove the dried kernels from each ear of corn, but don't mind that) are FREE and the scratch that i have been buying at 
TSC contains little cracked corn in it. Or least not enough to my liking. Another thing I started doing is "lacto-fermenting" the scratch grains so that the birds 1.) Eat it all gone; 2.) Consume beneficial bacteria that can aid in their G.I. tract efficiency, or so I think. I use the whey portion of my milk KEFIR that I ferment from whole milk. They go bananas over this fermented product!!
 
YMMV, and so will each of ours depending on our individual situations. I have half as many birds (19) as you do and mine free range for at least 12 hours per day. I'm prepping for Fall/Winter when I assume they'll need fat calories for heat production, so they will have layer pellets and a bit more cracked corn available to them during the colder months. One other thing that I am seriously considering, an opertion that we all can do, even in Winter, is indoor farming of meal worms. 
 
@ raerae2: My neighbor up the road told me of his similar experience with a local feed mill and abandoned buying any feed from them. The pricing at 
TSC for Purina and other brands of crumbles, pellets and scratch and cracked corn made buying from the mill directly cost-prohibitive, or so he claimed. I've never purchased from a local mill, so I don't have any direct experience with that. 
 
I like your use of the bird livestock to "run clean-up" after the horses have slobbered up their area! Hey, it also serves to minimize the migration of rodents and other "moochers" in and around the areas where your domesticated critters inhabit. Good job!
 
Barred Rock Cafe