An Economic Philosophy of Software
Direct Economic Factors
Software products are often created or extended as fixed-duration projects. The estimated cost of the project is compared against the estimated revenue generated, and if the balance is favorable then the project is given the go-ahead. Advanced project funders will consider protected revenue (how many customers will not jump to a competing product if this feature is added) and opportunity cost (what work could we be doing if we decline this work), factoring those into the decisions about the project.
I mentioned Barry W. Boehm and his book, Software Engineering Economics—http://books.google.co.uk/books/about/Software_engineering_economics.html?id=mpZQAAAAMAAJ&redir_esc=y in Chapter 9, Requirements Engineering. He introduced the idea of human economic factors; assigning a dollar value to the satisfaction (or otherwise) the software would bring to its users, for example. I'll come back to that in the next section, on Externalities...