I have read the posts and know you have thought this through alot more than I have.
My 2 cents: It seems to me that if you kept a flock of birds around, and just culled when neccessary for the dogs to eat, it would be alot cheaper and easier than doing 3 sets of 20 meat birds. With the cornish cross you would be butchering all at once, then freezing and storing the birds until you needed it. This way they could also have the guts, which is part of the raw diet from what I understand. I may be wrong there, because I have only done a cursery study of the idea, not anything intensive. You say you don't mind if the birds are old and tough for the dogs to eat, and this would also give you fresh meat to feed them.
Back to your initial question on cost:
Quoted from a thread started by AbbyDog: Frist Batch of CornishX - ...
I figured that the 10 birds used about 120 lbs of feed in 47 days. This is quite a bit less than what I was expecting So, I got 10.4 lbs of carcass meat (not including giblets, neck, feet etc) for 36 lbs of feed- given our local price of $34/ 50 lb bag of 22% organic chick starter, I estimate my chicken dinner will cost $2.35 a pound, based on feed alone...
Also, a good thread from DianeS: her final post has all the info on her birds.
https://www.backyardchickens.com/t/673395/my-cost-analysis-for-raising-my-meat-chickens
How much is grower/finisher going for at your feed store? According to AbbyDog, it looks like about 12 pounds of feed per bird to butchering age. DianeS had 17 pounds feed per bird.
Editted to add: I believe those were for Cornish Cross chickens, which are the best for feed to meat conversion. It would be more for a dual purpose breed raised in the same conditions. However, it seems like the dual purpose breeds forage better and can glean some of their feed from the environment if allowed to free-range, so they may end up with less purchased feed per bird.