I played around with some figures a few weeks ago when I had only two chicks hatch and could not justify getting a whole bag of feed (it would have gone bad before they ate it all), so I tried this on them. They were barnyard mixes - not the fast-growing Cornish cross. But they did just fine on this feed and were feathering out nicely.
This has mix is 20% crude protein. It is a five-pound recipe.
1 pound whole white millet
1/2 pound chia seeds
1 pound wheat germ
1/2 pound nyjer seed
1/4 pound poultry premix appropriate for growing birds (not layers)
1.75 pounds brewer's yeast
I would mix this with some water or milk, and give them chopped up dandelion leaves, lambsquarters, plantain, etc., from the yard. I used sand for their litter, so they had plenty of access to grit.
These were things I had around, but note they are all very small seeds and things. IDK if I would go out and buy chia for my chicks in the future, but I had a bunch for personal consumption and thought they'd like it. I only had to make them one batch until the rest of the chicks hatched or were mailed, so everyone's on bagged starter feed right now. If you plug prices into the spreadsheet, you come to more than $1/pound! So unless you have these items lying around your shed for one reason or another (my sister's finch died, so I had the nyjer seed she had given me), it is not very cost effective. But I wanted to give an example of how you can mix things together to get a good ration. Diversity is key, as well as access to fresh pasture & bugs.
I may try raising a batch of chicks on a similar ration, a more cost effective one, later in the season. But I have a lot going on right now. It may need to wait until next year.