So I've mostly only bought the core rulebooks because I was under the assumption that any creature created for modules/campaign settings inevitably make their way into the core bestiaries.
However, a few of this month's modules have grabbed my interest but was wondering if it'd be a worth-while purchase (the content in it won't be reprnted 2 years down the line).
I'm reasonably sure this idea is untrue. You might see select monsters get reprinted in the next bestiary, but I've bought the anniversary edition of RotRL and it gives unique rule sets you'll never find in any of the rulebooks. Not to mention the player's companions, which I guarantee contain things you'll never see in the rulebooks.
Even just focusing on monsters in the bestiaries, that is not the case. Paizo's homebrew (e.g. not OGL from some other source) monsters may only live in an AP volume and will never be printed in the core line, and other monsters that are licensed from places (such as Chaosium) similarly will never be released in OGL form in a core book.
See also this post by James Jacobs.
It's always a good investment to buy the adventure paths. They are generally packed with informative gazetteers and bestiaries that are not always reprinted elsewhere.
Following up on this, whether or not monsters in a particular book are OGL seems to vary; as a general rule the monsters usually are OGL from glancing through a bunch of books I have selected at random from both the Adventure Path and Campaign Settings lines.
As an example of the licensing deal I mentioned above, though, Adventure Path #46 Wake of the Watcher says the following in its OGL declaration:
Product Identity: The following items are hereby identified as Product Identity, as defined in the Open Game License version 1.0a, Section 1(e), and are not Open Content: All trademarks, registered trademarks, proper names (characters, deities, etc.), dialogue, plots, storylines, locations, characters, artwork, and trade dress. (Elements that have previously been designated as Open Game Content or are in the public domain are not included in this declaration.) The Dark Young of Shub-Niggurath, Dimensional Shambler, and Gnoph-Keh are specifically designated as Product Identity.
If you like NPC statblocks, those won't be repeats no matter what since the NPCs are never OGL in any case (they're characters). Monsters in the bestiaries may be reprinted if they aren't excluded like above, but as you'll see below that isn't all that common. For giggles, I opened up a rather old AP (Curse of the Crimson Throne), which was released before the Pathfinder RPG was. As such, stuff in the back had 5 Bestiaries worth of pages to get reprinted in, let's take a look at how many made it:
- Carrion Golem - Bestiary 2
- Devilfish - Bestiary 2
- Dream Spider - Not reprinted
- Doll, Soulbound - Bestiary 2
- Raktavarna - Bestiary 3
- Reefclaw - Bestiary 2
- Daemon, Leukodaemon - Bestiary 2
- Daughter of Urgathoa - Not reprinted in core line (reprinted in Inner Sea World Guide)
- Giant Fly - Bestiary 2
- Giant Maggot - Bestiary 2
- Lawgiver - Not reprinted in core line (reprinted in Inner Sea Gods)
- Beatific One - Bestiary 3 (as Asura, Upasunda)
- Rajput Ambari - Not reprinted
- Rakshasa Maharajah - Bestiary 3
- Sikari Macaque Swarm - Not reprinted
- Bonestorm - Not reprinted
- Cinder Wolf - Not reprinted
- Havero - Not reprinted
- Red Reaver - Not reprinted
- Danse Macabre - Not reprinted
- Gug - Bestiary 2
- The Prince in Chains - Not reprinted in core line (reprinted in Inner Sea Gods)
- Umbral Dragon - Bestiary 2
- Akaruzug - Not reprinted
- Belier Devil, Bdellavritra - Bestiary 2
- Contract Devil, Phistophilus - Bestiary 3
- Mobogo - Bestiary 3
Of 27 monsters printed in the AP (technically 29, but I excluded those two from the above listing. One was Achaekek but gods in Pathfinder don't have stats, the other was reprinted as a Haunt instead of a monster), only 15 were reprinted in a core line Bestiary, or 56%. 9 (33%) were not reprinted at all, meaning that they require conversion from their 3.5 versions as found in the books to be usable in the Pathfinder RPG.
Looking at a more modern AP, Shattered Star, we get the following:
- Agathion, Bishop - Not reprinted
- Fleshdreg - Bestiary 4
- Fleshdreg Swarm - Not reprinted
- Melfesh Monster - Not reprinted
- Skvader - Not reprinted
- Wolpertinger - Not reprinted
- Snallygaster - Bestiary 4
- Bogwid - Not reprinted
- Bogwiggle - Not reprinted
- Grand Defender - Not reprinted in core line (reprinted in Inner Sea Gods)
- Bronze Sentinel - Not reprinted
- Marble Sentinel - Not reprinted
- Caulborn, Chrestomath - Not reprinted
- Clockwork Familiar - Bestiary 5
- Fachen - Bestiary 4
- Polevik - Not reprinted
- Cephalophore - Bestiary 4
- End's Voice - Not reprinted
- Kyton, Ostiarius - Bestiary 4
- Psychopomp, Kere - Not reprinted
- Qlippoth, Gongorinan - Not reprinted
- Bhole - Bestiary 4
- Flying Polyp - Bestiary 4
- Kurshu the Undying - Not reprinted
- Nightguant - Bestiary 4
- Azata, Raelis - Bestiary 5
- Clockwork Dragon - Bestiary 4
- Living Rune - Not reprinted
- Mezlan - Not reprinted
Of 29 monsters printed in the AP, only 11 (38%) were reprinted in a core line Bestiary. 17 (59%) were not reprinted at all in any other book, making the AP bestiaries the only place to find them in print. All are OGL so they can be found online, however.
Note: My stats for where a particular monster was reprinted were pulled from the copyright blocks on d20pfsrd, as that was far easier than searching a bunch of books for the monster name each time.