I'm good with 500 pages, but only just. Unless, of course, it's something I was going to read anyway.
On availability -- I routinely check availability and price in two different public libraries and three different Amazons -- US, Canada, and the UK. I buy where it's effectively cheapest, unless WhisperSync is a major consideration. (It is, for long books that I want to be able to go back and forth with.) If WhisperSync is involved, my only two options are US and UK -- I don't have an Audible account in Canada. Yet.
As for PD works? Well, the price is right. But my sense is that they run the risk of not being as successful as we anticipate. (Whose Body and T3M come to mind immediately.)