I would be highly surprised if you retained an 80%+ weight on a butchered human. I'm an exceedingly careful hunter, keeping parts of the animal most don't (I keep the heart, lungs, liver, kidneys. Most hunters don't) and after butchering I am lucky if I get 60% weight to food ratio...
I figure ghouls are not overly fussy and all of the organs are fair game.
It is a shame they can't have it cooked because liver is sooooo much better when fried IMO.
Remember, its FLESH you need. That means muscle tissue. Fat is not tissue. In fact eating straight fat is a good way to make your self sick! Also, don't confuse marbling with fat... they are different as well...
Ghouls are still living organisms though and probably need some level of fat in their diet. Granted you wouldn't just go snarfing a lot of it straight, but you can still chop it up and mix it with the rest for flavour. Indeed Suet (raw beef or mutton fat -including the hard fat found around the liver and kidneys is still a key component of haggis and similar dishes, so you are not losing all the fat.
Edit:
That said, we did mention
here how a human skeleton would account for 12% -15% of body weight and fat does count for 12-17% on the average person which like you said a lot of that fat probably is not going on the menu.
As I mentioned before ironically the best meat is on the lean runner type with the lowest body fat that is also unfortunately the hardest ones to catch.

This also raises a good point.
We automatically assume that bones are out, because the requirement says flesh. However would bone marrow count?
I mean it's not a lot, but hey some people like crab legs.
Hmm.. note to self: possible market for bone crackers for the ghoul market. Have Roddy see what kind of pressure we are talking about and can a manual device do it easily.
Unless you have actually butchered an animal by hand, its hard to imagine just how much we don't/can't use. Some of it becomes toxic just from the processing (anything related to digestion you want to stay clear of.... fical matter can spread easily).
Yes and no. We do have some idea as we did our share of hunting in earlier years, but having been abroad as well, I recognize Western hunters do not use as much of the kill as some hunters in other corners of the world.
Tripe (beef intestines) are often eaten and many cultures eat animal intestines. Indeed the French still eat Andouille of which some varieties use the entire pig gastrointestinal system.
Granted in the later case this is smoked, but it does not mean a dry cure could not be done that could fit a ghoul's requirement with a bit of culinary magic.
Plus like the requirement says, Flesh. So they don't need to be skinning a dwarf (though I would insist on washing one) before eating, compared to where you wouldn't keep the hide on a cow or deer for consumption.
Mind you, I am not going out and butchering a person to find out :p
Awwww..... *puts away the Knife and Scale set*
My only real question though is: 'where would ghouls get the money to pay people for their bidies???'
Yes there are lots of people in the barrens who might be tempted by such an offer.... but how are the ghouls paying? They can't hold real jobs, they are hunted in many countries, and they are not exactly welcome in the workplace.... so where are THEY getting that extra 2 to 5k a year to pay for a body when they themselves live in the barrens?? If anything, thry are worse off then the SINless in the barrens. At least the SINless can actually look for a day job without worrying about getting shot for bounty money....
That is a good point, though a smart ghoul might well offer their.... cleanup ..... services to the local criminal organizations and get some bodies/favours/gear that way.
Or a team up, I recall in another thread we were discussing a Wendigo using his abilities to hunt and drain bodies and letting his ghoul allies eat the remains, plus probably being unknowing scapegoats if anyone comes looking for the missing people.