Considering the length of time over which Opeth has sold albums, they probably have made very little money per member from album sales. Remember artist royalties on album sales are shocking, no matter if you are Opeth or U2. If Opeth are getting more than US$2-3 per sale, I would be surprised. No doubt, like all bands, they make most of their money from touring and merchandise. Of course, publishing rights add some money to the pot. I would guess that they make an OK living doing something they like.