I had a flock of 40-50 Barred Rocks that I raised when I was a kid. I raised them from babies to egg production/butchering on nothing but free range and scratch feed with oyster shell available in a dish when they wanted it. We didn't know much about them then, it was just an impulse buy when my dad took me to the feed store once, I loved the baby chicks so much that my dad bought me a big box of them. The feed store clerk told us they ate scratch feed, so that is what we fed them. The hens laid pretty much daily year round for us and as far as I remember were healthy. We had tons of double-yolk eggs too.
Currently I am in a similar situation to you. My husband and I lost our jobs in February and have not found work since. We are both in school, paying for that out of our own bank account because financial aid is not available for us. We just sell things we own or do odd jobs now and then to feed our animals. I feel very strongly about raising as much of our food as we can and being self-sufficient, not dependant on the government and grocery stores to feed us. We don't like to eat a lot of processed food and chemicals that are unhealthy. Homegrown food is much healthier, and being Jewish in a very non-Jewish area (we get lots of nasty stares in public!), kosher food products like meat or cheeses are impossible to get unless we make our own or drive several hours away and pay prices we really cannot afford. We have dairy/meat goats as well as chickens and a garden for vegetables. Other goat farmers get all over me for not feeding my girls multiple pounds of dairy animal grain mix or sweet feed every day, but not only do I feel it is unhealthy for them to eat so much grain, I just plain can't afford it, at $18 for 50 pounds of sweet feed. Living in a state where there is very little water to grow crops, and hay/grain must be shipped in from out of state, things are very expensive. Hundred pound bales of hay run between $6 and $15, and it fluctuates wildly. Grain of any kind is $10+ per bag. 'Layer' feeds for chickens or even gamebird feed are not available here. The chickens get cracked corn (it's the cheapest grain available) as a treat--mostly just to get them to come when I call them--and have a bowl of brewery mash grains available to eat as much as they like (I can buy 55 gallon barrels of it for $15 from a nearby brewery, so it's economical!), and free range, plus kitchen/garden scraps for them and the goats both. My hens are not laying right now because of the weather, but they seem healthy and in good condition. I give them crushed eggshells for calcium, and they also eat a lot of scraps of hay that my goats leave.
'Back in the day' they did not have all the special formulated diets we feed today. Maybe their chickens did not live quite as long as they might today. But they did all right. Chickens in less developed countries produce fairly well on whatever they can find in the yard.