Ok then, the better phrase would be cost per unit sold...Let's ignore pathfinder for a moment....As gameprinter mentions, you still have a cost to produce a PDF so why wouldn't you have a cost per unit sold?
The printing companies (hereafter called "the printer" or "the printers") that Paizo and Wizards work with (at least, they did when I worked at Wizards) actually have you create a high-res PDF rather than a print prototype. The printers make the book from this PDF.
So, cost to produce high-res PDF for the printer to turn into a book = $X
Cost to produce low-res PDF for sale-download: two clicks, net cost $0
So for people who make printed products, the cost to produce a downloadable PDF is effectively zero.
Even if you're just using RPGnow.com or Lulu to make POD versions of your books, you're basically giving them your high-res files, which you already had to produce to make the product in the first place. It's not really an added cost.
Now for some personal evidence. My book Curse of the Moon was published as a PDF in May 2006. It sold about 100 copies on RPGnow/DriveThruRPG in the first three months (generally the peak time for a new book), and it's generally sold anywhere from 1-5 copies per month every month since then. So, for a book that I published over three years ago, it's still generating a trickle of income... income that doesn't cost me anything in terms of time, warehousing, or printing. It's free money, and in theory five years from now it could still be selling. As this wasn't a print book, my cost is paid of purely from PDF sales (there really isn't a cost per unit because I'm not creating a specific number of physical units among which I'm distributing my cost), with the expectation I would sell Y copies in a certain period of time. That cost was paid off sometime in those first three months. And even if I made COTM available in print-on-demand from Lulu.com or whatever, my cost per unit would still be zero because all I'd have to do is send the high-res PDF to the POD company (cost = none, it's been paid off) and start generating free income there, too (because the sunk cost for the book is paid off and as long as I'm pricing my POD book above how much it costs to print it, I'm making a profit).
As someone else pointed out, the cost of making the Pathfinder RPG Core Rulebook is sunk into the price of the print version. PDFs of Paizo's print products can't be "loss leaders" because calling something a "loss leader" implies you're losing money selling it at that price. PDF sales of print books are just free income on top of your expected income from the print run; you can't be selling something at a loss when the cost to make it is $0. And Paizo's set the PDF price of the PFRPG Core Rulebook low in order to encourage people to try the new game, people who might otherwise balk at a $50 hardback. And if those people like it, they may buy the print version--either directly from Paizo.com, or from their FLGS.
(Disclaimer: I work for Paizo, if ya didn't know that.)
(Also: Go buy COTM, it's a good book.

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