We recommend you put a link in the About the Book section, like is done at https://leanpub.com/rprogramming:

"If you are interested in a printed copy of this book, you can purchase one at Lulu."

Leanpub does not earn any revenue in this situation from the print books, and we're totally fine with that.  If we wanted to earn money from print books, we'd either (a) sell print books for you, by using Ingram Spark or Lightning Source in the background, or (b) charge money for our print-ready PDF export.  (We currently do not do either of those things--print-ready PDF export is available in all plans, including the free plan!)

If, when you click the print-ready PDF button, you think "sheesh, I wish that had cost me at least $79 or $149", what you can do is go to https://leanpub.com/shelf/buy and buy a 1-month or 3-month spot on The Shelf instead. That way, you get the added benefit of being on The Shelf!