If you read the nitty gritty of the PFS Campaign Rules you'll notice
that part of the “core assumption” is the Seeker of Secrets book.
It's the book dedicated to Pathfinders, and because of that, it's
fully open and legal.  Further, it's special status as core assumption
means it is assumed to be at any table playing, just as the Core
Rulebook, so a copy isn't necessary for you to be able to use the
contents of the book.

It is a good book, and well worth picking up if you want to get a lot
more flavor on the vision of what the campaign is supposed to be
about.  What I want to look at are the cheap Ioun Stones that can be
found towards the back of the book.

WHAT IS AN IOUN STONE?

First, what the heck is an Ioun Stone? They are little stones that can
confer magical abilities when one “wears” them.  You do this by
tossing it up on your head and then it orbits your head like the
planets around the Sun.  You can have lots and lots of Ioun stones
orbiting your head if you like.  Once their it's hard to have them get
knocked away.  The stones will change their orbits, avoid things.  The
only real way to get them down is to grab it yourself, which is just
an act of will on your part, or someone else needs to try and snatch
them from you, but this is pretty hard to do.

PRIMORDIAL D&D

Where the heck did the idea of the Ioun stone come from?  It sounds
weird doesn't it?  Ioun stones are old old school D&D.  They in fact
pre-date D&D.  They were part of the Jack Vance's “Dying Earth” series
of stories that helped to inspire Dungeons and Dragons.  You've heard
of “Vancian Magic” on message boards?  That's where the whole idea of
wizard spell preparation comes from.

The Wizards in the Dying Earth series were obsessed with Ioun stones.
Any rumor of a cache of them drove them to plot and maneuver and even
kill to get at them.  For the very powerful wizards, they were really
the only currency they cared about.  One hint lead a group of them on
a space faring journey in what best can be described as a magical
space ship to a far away planet.

IOUN STONES ON THE CHEAP

Anyway, there are lots of Ioun stones and most of them are very
expensive, and thus not all that interesting unless you are drowning
in gold.  However, with the publication of Seeker of Secrets, they did
offer up quite a few cheap (2000gp or less is my definition) Ioun
stones that low level characters, or any level, might be interested in
using.  The reason why these are cheap is because they have a crack in
them.  The perfect Ioun stones are a lot more expensive.

It should be noted, Ioun stones don't take up a head slot for magic
item use.  One of the reasons why they are awesome.

Seeker of Secrets Cheap Ioun Stones:

• Dark Blue Cracked: +1 competence bonus on Perception and Sense Motive checks 400 gp
• Dark Blue Flawed: +2 competence bonus on Perception checks and a –1 penalty to initiative checks 300 gp
• Deep Red Cracked: +1 competence bonus on one Dexterity-based skill 200 gp
• Dull Gray: No powers, but if you want something orbiting your head then 25 gp
• Dusty Rose Cracked: +1 competence bonus on initiative checks 500 gp
• Emerald Cracked: 1 temporary hit point (stacks) 2,000 gp
• Gamboge Cracked: Immunity to one kind of poison 1,500 gp
• Incandescent Blue Cracked: +1 competence bonus on one Wisdom-based skill 200 gp
• Iridescent Cracked: +4 competence bonus on Constitution checks to hold breath 500 gp
• Mossy Cracked: +1 competence bonus on one Knowledge skill 200 gp
• Mullberry Cracked: +1 competence bonus on Bluff and Diplomacy checks 400 gp
• Nacreous Gray Flawed: Protects against the appearance of aging 1,000 gp
• Opalescent White Cracked: Weapon familiarity with one weapon 1,500 gp
• Orange Cracked: Add one 0-level spell to user’s spells known or prepared 1,000 gp
• Pale Blue Cracked: +1 competence bonus on one Strength-based skill 200 gp
• Pale Ruby Cracked: +1 competence bonus on Stealth checks 200 gp
• Pink and Green Cracked: +1 competence bonus on one Charisma-based skill 200 gp
• Scarlet and Blue Cracked: +1 competence bonus on one Intelligence- based skill 200 gp
• Turquoise Cracked: +1 competence bonus on Ride checks 200 gp
• Vermillion Cracked: +1 competence bonus on Acrobatics and Swim checks 400 gp
• Vibrant Purple Cracked: Stores one spell level, as a ring of spell storing (minor) 2,000 gp

Advanced Player's Guide Cheap Ioun Stone (note that this isn't Core
Assumption, you need the APG to use this):

- Ioun Torch:  This item is merely a burned out, dull gray ioun stone
with a continual flame spell cast upon it. It retains the ability to
float and orbit, and allows the bearer to carry light and still have
his hands free. 75gp

Rules on Ioun Stones from the PRD:

These crystalline stones always float in the air and must be within 3
feet of their owner to be of any use. When a character first acquires
a stone, she must hold it and then release it, whereupon it takes up a
circling orbit 1d3 feet from her head. Thereafter, a stone must be
grasped or netted to separate it from its owner. The owner may
voluntarily seize and stow a stone (to keep it safe while she is
sleeping, for example), but she loses the benefits of the stone during
that time. Ioun stones have AC 24, 10 hit points, and hardness 5.

So toss some gold down and soon enough you'll have a bunch of rocks
floating around your head, giving you little perks and telling the
world you're a Pathfinder!