All of the advice already given matches with my limited experience: about half of the live weight is meat. I think BBW have a higher proportion of meat/other stuff. Our 8-9 month-old BRs yielded slightly less than half of live weight.
And, also repeating what other people have said: if you grind all the meat, including the breasts, you would get approx 15 lbs. of burger from a 30 lb. turkey.
My preference is to integrate some of the fat (from around the gizzard or up in front of the breast) into the ground meat. That makes it "self-basting" and less prone to drying out. I also like to grind the heart and liver into the meat, to give it a little more flavor. The only problem with doing that is it means no liver pate and no marinated heart shishkabobs.
Re: chickens and turkeys together: it's suggested that you don't let them mingle, because of the risk of blackhead. Having said that, a lot of people do just fine by keeping them together. If you called a local vet or your county extension service, they might be able to tell you how common blackhead is in your area, and that would help you decide if it's risky in your neighborhood. Blackhead is rare in my neck of the woods; therefore, lots of people keep mixed flocks.