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:

I can't tell you about price since I was given a kilogram of blue Scottish mussels but I can tell you about the meat % of total. I had guessed it would be about 20% but I was wrong. I weighed the shells next day using the local supermarket's digital scales & the result was only 428g so the meat was a massive 572g ~ 57% meat. I recommend them.

Toronto, Canada: 2#/908g PEI rope-grown blue mussels (per tag, packed 25 Sept, best by 9 Oct), Ca$4.49 which has been the local "on sale" price for about the last year. Cooked today. Shells = 428g, approx 47%, incl uncooked discard of 2 dodgy ones & one 1/2 shell. Flesh & broth approx 53%. Sipped off most of the broth, moistened flesh approx 180g, so about 20%.