What is the proportion of edible meat in whole live mussels?
The Tourism PEI site says that one pound of mussels is 20-25 mussels, about one cup of meat, and that you can serve one person that for a main meal or two people for an appetizer. 

The Food and Agriculure Organization of the United Nations says:


  One bushel of whole mussels should yield from 6 to 9 pounds of cooked
  meats. Percentage yield by weight may range from about 8 per cent to
  as high as 20 per cent of the whole mussels.


Finally, this is 500g of mussels (my lunch in Brussels on a hot day - my companion has the same):


And this is how I can remember it was 500g: