I have six standards and feed Purina Flock Raiser, homemade scratch of oats, wheat, BOSS and, in the winter, corn. I buy a bag of Flock Raiser about every two months, 25# each of oats and wheat every five months; the BOSS I have on hand for the wild birds so it's hard to say what that costs.
I feed moderate treats; yogurt, safflower seeds, veggies, etc., and they have food in front of them all the time.
So, probably I'm in the $20 per month ball park. But I save in pest control stress, I sell enough eggs to make up the cost of the feed and I didn't get them for money making operations. I like the six differently colored life forces strutting about the yard.
I like the higher protein of the Flock Raiser because I think they eat less and poop less. My neighbor feeds his birds, six standards, 16% layer and goes through a bag a month and his pen smells.
You can ask the local grocery store and butcher shop to save you scraps and produce to feed them that may offset some cost, but you'll have to do some nutritional research.
Mary