Figure on about 4 oz per bird per day for the layers, once they grow up. I never had meat birds, so I don't know.
I have dual purpose chickens now, but just hens. How much feed ultimately costs you depends on how you choose to feed, how much free-ranging your situation safely allows, and how creative you are in finding free, healthy foods for your flock. I do all three, but don't mess with the babies, they are too fragile. At 10 weeks, I start introducing the whole grains my adult birds get, and have them switched over by about 14 weeks. You need to know how to balance protein and nutrients to take this type of feeding project on, so it is definitely not for everyone.
My goal is to ultimately purchase only half my feed. With extensive free-ranging, this was the case for me all spring, summer, and fall, with no decline in production. Lots of big grasshoppers and beetles in my field, though....at least there were a lot....
A lot of feed is wasted to rodents, too. I have had as many as three chipmunks run BETWEEN MY FEET while I was throwing grain to my girls, and many more scurry in and fill their little cheeks. So I hand-feed my girls, throwing feed 2-3 times a day. I hang a feeder if I will be working late or if I go out of town and a friend just has to change the water and pick up the eggs. My feed bill went down noticeably when I started this. It is more work, of course, but here we call it "chicken therapy!" My DH and I try to beat each other to the feeding chores, we both enjoy it so much.
Have fun with your chickies, and do lots of searches to figure out how you want to proceed!