But here's the problem: the very concept of "marginal cost of production" is nearly made obsolete by computers and the Internet. It used to be that the effort to produce the copies was proportional to the number of copies being made. Not any more.
Actually, marginal cost is not made obsolete. It is very much applicable. But a low marginal cost and the larger audience that the Internet affords should dictate a lowering of the selling price.
The RIAA and its ilk are interfering with this natural process by keeping prices artificially high. The ease at which you can pirate a book or CD today is an opportunity but the incentive to pirate should not be coupled to that.
(Why else would we have spam?)
Spam is not a product for sale and is only part of the cost of whatever is being sold, so I'm not sure why you think that is relevant.