lots of things are cheaper but may not have all the nutrients needed...there are some recipes for amking your own feed if you have the ingredients available...free ranging helps but isn't always an option
If you have a local mill you can get 100 lb. bags of fresh ground hog mash or corn and mix it with the layer and scratch feed. Also anywhere that grounds corn mill will have mill that they can't sell for human consumption and thats cheap when you can find it.
If you are just looking for a supplement, you can give them cracked corn, which my chickens love and they sell scratch grains you can give them. Those are more like treats instead of something thats avalible to them all the time.