Not so fast...

If you sell 10,000 copies of a paper book that costs $40, chances are you get $2 per copy (10% of the publisher's $20 in revenue), leaving you with $20,000 royalties. Not bad.

If, however, you sell 2,000 copies of a PDF at $20 copy, and you get $16 per copy, you get $32,000. That's better. Furthermore, that assumes $0 in print royalty revenue. Similarly, if you sell 3,000 PDFs, and you get the same $16 per copy, you get $48,000. Much better. Plus, with print books, there's the annoying aspect of returns, etc.

Don't be so sure that PDF books should be a marketing tool for paper books. I like to think about paper books as a marketing tool for PDF books: if you sell 1/3 as many PDFs you're more than twice as far ahead...