So what's your definition of "pay to preview"?
A fair portion of it is the sniff test: Does this thing I just paid money for smell like a demo?
But here's a good test:
(1) Does the book prominently and/or repeatedly tell me that I should go buy some other book?
(2) When I go to buy the other book, will I discover that this first book that I bought has been rendered completely obsolete? Could I have skipped the first book entirely?
The Essentials Starter Set completely fails this test. The rules it contains for character creation are essentially non-existent and the level-up mechanics are basically equivalent to those found in the D&D boardgames. Simultaneously, the DM isn't given enough rules to really run anything except a combat simulator. (And not a particularly interesting one, since he's only given 16 monsters.)
In addition to not providing a complete RPG, the Essentials Starter Set prominently and repeatedly tells you to go buy
Heroes of the Fallen Lands,
Heroes of the Forgotten Kingdoms, and the
Dungeon Master's Kit. And once you've bought those books, you'll discover that your
Starter Set is irrelevant and can be stuck in a closet.
This can be contrasted with the Mentzer
Basic Set: Although it might benefit from including a couple more levels of play, it includes a complete set of rules for low-level adventuring.
Like the Essentials
Starter Set, the Mentzer
Basic Set mentions additional books that you can buy when you're ready for more: The Expert, Companion, and Master Sets. Also modules B1 thru B5. Module M1. Dungeon Geomorphs. Player Character Record Sheets. Monster & Treasure assortment. But when you go to buy any of these books, you'll discover that they don't actually replace your
Basic Set: They supplement it.
AD&D is also mentioned, and those rules would functionally replace the
Basic Set. But you're not told to go buy it. You're specifically told that AD&D is a different game. So even if you do decide to buy AD&D and put the
Basic Set in the closet, everything about the Basic Set -- in both word and deed -- says that you're choosing between games, and not merely putting the demo away to play the real thing.
The question with
Pathfinder Basic is the degree to which you can package a product which convinces me that
Pathfinder is a different and more advanced game that I can choose to switch to, but don't need to switch to in order to continue playing.
I'll be honest in saying that a company focused on selling adventure paths that take you to levels well beyond 5th may have a difficult time selling me on that.
OTOH, full rules for 1st to 5th play would give a fairly comprehensive experience. Coupled with proper presentation, that might be enough.
Making
Pathfinder Basic into an E6-type product might be an interesting way to go, but also a fairly radical one.