Beef rib roast - how much do the bones weigh?
The key factor is going to be the breed of cattle. Angus, a very high quality beef, also has heavy bone structure and a less favorable bone-to-meat ratio, where as limousine has a lighter bone structure with more meat-to-bone. Other breeds will vary as well, with hereford being (IMHO) the 'happy medium'. But, of course, each individual steer is going fall with a range of possible values rather than a constant factor. 

Such value propositions really come down to individual taste and priorities. I suggest to that the comparison is really not just a xLb of bone-in vs. yLb of bone-out. The quality of getting bone-in (again, IMHO)is far superior. The attached rack provides for a degree of insulation and allows for a slower roast protecting the whole rack from over-cooking (a sin to be sure). 

If you are stuck on just 'doing the math' if you are looking at a whole (7 bone) prime rib, the rack, the bones, along with the meat &amp; fat attached to the bone that would remain if the rack were to be removed, will account for about 15% of the overall weight (give-or-take all of the variables mentioned - and more). For something 'more or less' than the whole prime (ribs 6-12) as you can see the bones will vary in size depending on whether you are at the short end or the large end of the rack. 



This article, from Weber, offers a good deal of analysis of the rib roast in general and may provide some good additional information for you. 


  Bone-In vs. Boneless
  
  Beef rib roasts are sold two ways—bone-in or boneless. There are
  several advantages to cooking a bone-in beef rib roast:
  
  
  Fat and connective tissue that surround the bones add moisture and
  flavor to the meat as it breaks down during cooking. 
  Rich flavor from bone marrow may migrate into the meat during cooking. 
  Bones slow the cooking of meat and limit the evaporation of moisture. 
  Bones are easily removed after cooking and make a tasty treat for the chef! 
  
  
  Buy a bone-in beef rib roast whenever you can.