We grow 'heritage' birds for this reason - for eggs AND meat. The Black javas are slow growing...typically we send cockerals to 'Camp Frigidaire' about 20wks old and get a nice 5# bird out of it. But that's nearly 14wks of crowing to listen to...and our neighbors also!
We grow meat birds periodically - because the heritage meat is not as suited to the grill or for roasting as we're used to. But the heritage meat is very good in soups, casserole dishes and such. So I find I use a lot more in the cold seasons than in the summertime.
We have grown Freedom Rangers quite happily several times now, just about every year - they act like a heritage breed, they just grow a LOT faster - about 13wks was processing time for the bigger boys; 15 wks for some of the slower smaller girls.
We have grown out a batch of Cornish X once - and I will not do that again. The birds were not attractive (not many feathers), they were not active and they really just laid around in the run most of the time. Toward the end of their life, they were not physically comfortable (panting when it was only 55 degrees?!), and they could no longer make it up the 14" run into the coop...so I built them a different house for the last 2wks of their lives. The quantity of waste was more than our compost system and our chicken run could handle. Remember, what goes in, must come out! But I do say the meat was the BEST roast bird I've ever eaten. But it was hard to swallow because I kept remembering what mucking out their coop was like while they were alive.
Each breed bird does have different growing ratios and texture too. We've realized our Swedish Flower cockerels have a different texture than our Black Java cockerels - both raised on same feed, same yard, same everything...just a different breed.
As for costs - I compared the costs between raising 25 Cornish X and 25 Freedom Rangers - and the difference was negligible. Because the cornish ate just the same amount as the Rangers - just a few weeks less between the two. But the Rangers were beautiful birds that loved life - ran around eating bugs, sunbathed, dustbathed, acted in every way like a 'regular' chicken...just a really fast growing version.
If you're going to process yourself that is another consideration to figure out. Youtube taught us the hows and whys - but it does take practice to become proficient in the techniques. Not hard - kind of like trying to get a toddler out of a tight-fitting coat - but it's not something we do lightly or without dignity.