For free ranging to help, it has to be virtually all day long. The fields where they range need to be rich in seeds, plants, grasses, bugs and insects that the birds catch, find and eat. The breed of bird makes a big difference in the skill required, as does practice. Free ranging can save a bag of feed, perhaps, for us from May thru October, but that is all. In other words, feed costs can potentially drop by 30% during that period.
But this area is not particularly fertile and the ranging season is relatively short. I don't honestly calculate the savings to be monumental, for us, for those reasons.
A flock of 20 full sized laying hens easily go through a 50# bag of feed, @ $14 a bag, every 12-14 days, if pellets constitute the sum of their diet.