You are completely right hapless. A good comparison is a dime bag to a candy bar. What adds confusion is that a dime bag is generally thought of to contain 1 gram these days. But this is not a candy bar and is not marked by weight. You won't find a "gram for a dime" bag that actually contains a gram. Hence why dealers are called choppers. They have to know how much to chop and still make money. Illegal or not, it can still be a slim margin business so these are accepted business practices by both vendors and customers.