I would guess that the folks down the road from us have about the least expensive, cash money wise, you can get. They don't feed grains at all.
The birds are in salatin type tractors that get moved 2x a day. The tractors look to be 8' x 16'. The each have water barrels attached to the tractor, with bell drinkers. The tractors are on an alfalfa hay field. The birds are butchered at 7weeks for fryers, 10weeks for roasters. AND they get the organic label.
The man I spoke to said they rotate the fields that the tractors are on every year. The field that had chickens on it the year before is super productive the next year. That hay is used for the beef cattle.
It takes them about an hour to move the tractors each time...4 people. So one would have to figure in wages for this endeavor...but if you're doing it for yourself....
Our own operation....my meat birds always went out with the other chickens and they foraged. The did get grain as well, but only because the forage wasn't all alfalfa. Mainly a timothy hay field, with some extra pea vines and turnips. I'd have to look it up, but as I recall it was almost 4:1 ratio.
Our freerange/pastured birds rarely eat much grain in the spring/summer months. Any of them.
The cost you put into your birds will depend on the amount of work you're willing to do. Some people insist on figuring in a $$perHour wage. If you do that, you're sunk before you start unless you start factory farming.
You can snag stale bread from bakeries often for $10 a truck load. that's wheat.
You can go fishing to give them fish to up the protein. If you live near a fishing resort, you can stop every night and pick up all the fish heads/tails/etc for free (they may even pay you). The resort can't just compost or throw them away...they have to dispose of them some other way.
You can raise worms for protein.
Ask all your neighbors/co-workers to bring you the carcasses from their roasts/turkeys/chicken dinners. The birds will love you. Also get eggshells that way for calcium. (personally, I don't do that because oystershell is so inexpensive, and during the summer they get calcium from the greens)