I really think it depends on your climate and humidity and the breed of your hens. I have never got any where close to 1/2, but have seen a reduction in the feed used. Mine are on the prairie, and in bushes and grass.
Various times, I have been hit hard with predators. I like to let mine out, I think they get better exercise and it is healthier than the limited space. However, not a significant feed source. The current rooster is a Bielefelder rooster, and he does not like to get to far from the coop. I like his coloring, size and shape, but I would like a bit more rummaging. His days are numbered. However in his defense, I have had roosters whose flocks covered a much wider area, and not a significant feed reduction.
It does depend on the production of the land. Most backyards, I don't think would supply much with limited bugs and plant seed. Even if the original poster does not spray and treat the lawn, most of the neighbors probably do.
Mrs K