Yes, you definitely want to crunch the numbers again. Unless you're paying $40 a bag of feed, it's going to cost you much less to feed them to eating size. We raised seven roos to 16-20 weeks. The 16 weekers weighed about 3-4 lbs after processing, the 20 weekers definitely had more meat on at about 5+ lbs each. These were a cross of a RIR over a White Leghorn hen.
The meat was absolutely delicious, tender, plump, perfect.
Remember that they're not eating anywhere near that 1/2 lb of food a day early on, they start out eating just a tiny amount and it gradually goes up over time as they grow, so you can't use 1/2 lb or even 1/4 lb of feed per day as your guide. Like everyone else, we found it was in the neighborhood of $4 or $5 per roo by the time they were ready to butcher. (That's if we're feeding them manufactured feed. For our own mix, it's less.) That's around $1 a pound for delicious meat that we knew exactly what went into it. The flavor of home raised meat is incredibly better. Another thing to consider in most store-bought chickens is that they inject up to 15% "flavor solution" into the meat during processing, so possibly 15% of the weight is just injected water, which ruins the texture and flavor of the meat as well. Can't believe they get away with that. Look on the package. You'll see that on nearly every package of any kind of meat at the store these days. Tiny print so few people notice or realize what it means.
As for the fast food scraps, there's going to be hamburger, lettuce, onion, cheese, etc, all of which are good for your chickens. Even adding the bun, I would bet that it's just as healthy to feed them that as to feed the processed chicken feed. If you read the ingredients of the processed chicken feed, you'll find it's pretty much the ingredient list that you'll find for a fast food meal, but without the greens. I don't know why processed chicken feed is considered so perfect. It ends up being that you're feeding your chicken processed food, just like eating processed food from the store with a vitamin pill added. Yay. Whoopee. (And don't get me started on store bought dry dog or cat food! It's just flavored cornmeal, a bit of soymeal for protein, with vitamins and a bit of white rice and maybe a bit of vegetable. Not a natural dog food!)
We started out feeding our chickens, guineas, and ducks the processed chicken feed. Most of it got wasted. They would eat a bit more of the Purina Flock-raiser than any other brands, but we had sooo much wasted! So we started mixing our own. I found recipes for homemade chicken feed online, what people used to feed their chickens. I kind of made up a similar recipe for our birds, and they love it, it's cheap, and they're quite healthy and laying up a storm.
I buy a bag of scratch (usually cracked corn, wheat, and sometimes other grains), a bag of rolled oats (from the horse section of
Tractor Supply), a bag of black oil sunflower seeds, a small box of flaxseeds, barley, sesame seeds, lentils, etc from the grocery store. We mix sort of equal amounts from the scratch and oats, slightly less of the sunflower seeds, a bit of flaxseeds and the other stuff, and then always have free choice grit and then some oyster shell for the hens. They also get whatever vegetable peelings (not potato) and trimmings we have, and they're in a chicken tractor outside so they get grass, weed seeds, worms, bugs, etc. I sometimes throw them in a cabbage to devour. It's also suggested to add in some powdered milk, which I do when I remember.
This costs about half what a bag of feed does for the same weight of our mix. Sometimes I'll buy a bag of wild bird seed and give them some of that, too, which they love.
Yeah, it's not "perfect," but who eats a "perfect" diet anyway? I'm sure someone could and probably has invented the perfect pelleted nutrition food for humans, too, but no one's going to eat that. Especially not for every meal. Given a choice, a chicken will eat what makes them feel good. They seem to feel good on this mix we make for them and they don't waste it, either. Anything that falls out of their feeder is easily scratched for, found, and gobbled up. Any processed feed that fell out was ignored and wasted.
Oh yeah, and for chicks, the grains and scratch are too big, I mixed cornmeal, regular quick oats from the cereal section at the store, and then some Purina Flock Raiser. Probably half Flock Raiser, 1/4 oats and 1/4 cornmeal, along with some playground sand from Lowes for grit. Probably not some people's idea of perfect, but they were all healthy, happy, strong, energetic, we didn't lose a single one, it worked well for us. I think we fed them this until about ten weeks, though I can't remember for sure.
Good luck. I say feed them the fast food scraps, buy some whole grains for them when they're a little older, (make sure they get plenty of grit to help digestion) and you should have plump, juicy, delicious roos to eat in about four months, and for about the price you pay at the store.