The price of one kilo of mussels (blue mussels) here in my town is around 60–70 SEK (9–11 USD). I think that makes a large enough portion for two people, but when checking if all of them are alive and when cleaning them, the price runs higher. I'm interested in what the price per kilo of the edible part (the muscle) of the mussels are. In one mussel, how much by weight is shell and how much is muscle? 
It depends on where you live, and what type of mussel you are eating  The blue mussel popular in Europe and North Pacific typically has an uncooked meat ratio of 25% to total weight, and 20% when cooked  The New Zealand Greenshell mussel (available flash frozen in Europe) typically has an uncooked meat ratio 55%, and 50% when cooked    Around US$2 per Kg. These will vary by season and supplier of course 