I'm gonna second the Flock Raiser AND skipping the corn altogether.
Oyster shell is cheaper than protein which is what you get extra instead of calcium and why those feeds cost more. Offering oyster shell on the side free choice SEEMS more expensive at the beginning.
Adding your general location to your profile will help peeps make the best suggestions possible at a glance.
That being said, have you shopped around at all? There is easily a $1-3 difference per bag depending on where I go.
Enough CANNOT be said about proper nutrition and what it may save you in illness, parasite and disease resistance and such. For me, corn is the same price as feed... and yes I have spent WAY too much over the years on feed, easily over $100/ month...
BUT I'm not spending $ to go to the gym or to therapy and it's way more beneficial than focusing on bad stuff or doing drugs!
Couple keys to consider.. raise feeders to be level with the birds back to help avoid waste. Make SURE your feed is locked up at night to avoid free loading rodents. Day time free loaders such as song birds may be an issue as well.
Selling hatching eggs or chicks or even started birds was the only way to cover any of my cost significantly (even them I feel lucky to break even on just feed). Which added a whole other fantastic experience... And expense

(cuz then came the incubators) Keeping roosters isn't free. The heavier the breed like Orpington or Wyandotte, the more they eat and lay less eggs than lighter breed like Leghorn. Choosing lighter breeds or even bantams can significantly impact feed consumption. Free ranging does also decrease consumption depending on pasture quality. With decent pasture it saves me about 10% verses not free ranging. But then you MIGHT lose a bird to day time predation negating any savings at all.

So many choice and some are very personal. Good for you for seeking to do the best for your birds and finding ways to do that!
My cost in just feed is about $4 per bird, per month. Figuring out your cost per bird might help you decide on a good amount to keep long term. I sell hatching eggs most often between $25- 65/dozen and eating eggs go for about $4/dozen. Chicks $10- $15 each, cross breeds would be MUCH less.
One way or the other... corn should be kept to no more than 10% of total daily ration as it does not have enough protein nor the added amino acids, vitamins, and minerals of any formulated ration. High energy low nutrient diets can cause an amplitude of not only health issues but also behavioral issues.
5 roosters is quite a few.. I would consider re-homing or sending a few to freezer camp. And also maybe re-homing either or of some of the older or the younger birds or some of both. I like to sell off a few older and bring in a few younger each spring to keep winter laying consistent with my family needs. Note corn does NOT keep chickens warm, their feathers do. It just provides calories (aka energy) and most formulated rations are already made of mostly corn.
I hope this gives you a few ideas, I see lots of good suggestions from other posters already!
