You raise an important point. It sounds logical that because Lipitor has such huge sales, margins would be greater than on smaller drugs. There must be various aspects to huge size that cut costs in manufacturing per unit, advertising and marketing. I don't believe the company has broken down margins by drug and analysts that have post-Lipitor expiration estimates may not have properly focused on this issue.
Takes about the same time & effort to sell a billion dollars worth of Lipitor to a major wholesaler as to sell $10 million of a non-blockbuster with a fraction of the demand. There are also of course enormous economies of scale in making & distributing the stuff.