@Thomas Shey - but that's not all the costs, and it's this other portion that people don't think about. You have costs for writing, editing, and at least art. You have to divide those costs against how many you believe will sell. I think I've put in
at least a hundred hours of writing into the most recent monster manual I've done and that doesn't include the art for some 350+ monsters (Because of art, I believe monster books are the most expensive to produce), plus a hardcover cost. A platinum seller on DriveThruRPG has to sell 1,000 units to get that ranking (the highest ranking tracked is Adamantine, which is 5,000).
This is just me spitballing, but assuming a "living" wage of $25/hour, with $25 for a quarter page piece of (professional) art and at least $100 for a cover, we're looking at a low side for the monster book above at $2,500 for the author and, ~$7,600 for art (for about 300 pieces) for a total of $10,100 just in raw cost (without an editor <cringe>, or a layout artist) for basically a one-man show*. Assuming it hits Platinum, that's a minimum of $10 raw per PDF (not counting DriveThru's cut, so at least $12 per to account for that).
* There are very few people I know who are skilled enough to both write and do the art for a product at a professional level. Normally, at least one of the two has to be sourced out.