It is fairly easy to get ALOT of free food for your chickens. Local restaurants (esp. 'mom&pop' type places) will willingly keep a 5 gallon bucket for you and fill it with food scraps. Your own food scraps are excellent (there is very little chickens cannot eat). Many of us are growing some of our own feed (corn, sunflower seeds, turnips, millet, wheat, oats, buckwheat, etc). Ideas are almost endless (I just learned you can feed chickens cow hearts...and since we are friends with someone who owns a processing shop we just hit the jackpot).
The problem, however, is how to provide your own feed WHILE SIMULTANEOUSLY given the chickens the proper nutritional ratios. At this point my own feed is supplemental. I do want to get to the point where I provide half or more---but I'm still learning about nutrition, so I can't help much there.