Just to add my few cents, I think it all comes down to how long they would take to mature and reach an appropriate age.I know that some breeds can take as long as 6 months to mature where as others as early as 12 weeks.
Your next question should be is the amount of meat you get worth the time it takes for them to grow and the feed costs over that period. Also a point on what Ridgerunner said, letting them forage does save on feed costs, but it should be noted that it is false economy expecting to grow big meaty birds on grass and grubs. This s seen with people who free-range commercial strains of broiler taking about 8-12 weeks for them to reach full size whereas in the commercial industry,it takes about 40 days.
By all means let them forage, in fact you'd be in my bad books if you didn't, its just they will take a little longer to grow if that's the way you choose to go.
And my final thing that I think you should consider is the actually killing part. I don't know if you've ever killed something you've raised yourself but it can be a daunting experience. My first time, I felt like I had lost a friend and was pretty upset. Don't let me put you off if you still want to give it a whirl, it is a rewarding life experience in the long run. Just a couple of things to keep in mind:
1.Plan ahead, you don't want to be left without something you need.
2. If you are "chicken" don't attempt anything until you are calm and confident in your actions. Only doing half the job will deeply disturb the bird and you don't want that, especially one that you have raised yourself.
Anywho, I can't think of anything else. Hope that I managed to provide something useful in all that rambling