Magic Item Creation Guidelines: +4 bonus to Caster Level
The Orange Prism Ioun Stone provides a +1 bonus to Caster Level, and requires no slot. Now, note that this does not specify that it is the Caster Level of one class; if you were a multiclass spellcaster with more than one spellcasting class (a mystic theurge, perhaps), the bonus would apply to all of your classes.
Moreover, an item that provides a bonus to Caster Level can be used by a Psionic character to get a bonus to Manifester level, by the rules of Psionics-Magic Transparency.
So a cerebremancer could use an Orange Prism to get +1 to both Caster Level and Manifester Level. That’s convenient.
Therefore, we want a +4 (or +6) version of the Orange Prism, but we’ll allow it to take a slot instead of having no space limitation.
The basic Orange Prism costs 30,000 gp.
Since Ioun Stones have no space limitation, they cost double what an item that does take up a slot costs, based on the Table: Estimating Magic Item Gold Piece Values. Thus a +1 bonus to Caster Level that isn’t an Ioun Stone should cost 15,000 gp.
Costs for bonuses are generally based on the bonus’s value squared, times some number of gold pieces. Since the Orange Prism is a +1 bonus, 12 = 1, so that number is 15,000 gp. So a +4 bonus would be
42 × 15,000 gp = 240,000 gp.
And a +6 version would be
62 × 15,000 gp = 540,000 gp.
Those are both enormous amounts of money; the latter takes almost all of the wealth of an 18th-level character (and most DMs do not allow you to put all your wealth in one item, and for good reason). Even at 20th level, the +6 bonus is more than half your expected wealth.
They are just guidelines after all, right?
The guidelines do not always provide a perfect value for all items; there are numerous cases where the formulas produce items that are much, much too expensive or much, much too inexpensive.
Here, though, it’s hard to argue with the forumulas. Bonuses to Caster Level are extremely valuable, and a +4 bonus is huge. They allow you to cast spells better than someone of your level usually could, which is just about the biggest deal, short of simply casting higher-level spells anyway. And with psionic augmentation, you basically are manifesting higher-level powers.
But wait, I’m a cerebremancer: I’m already behind
This is the big thing: to become a cerebremancer, you need at least three levels in each of the base classes, before taking levels of cerebremancer that advance both. Therefore, your Caster Level and Manifester Level are already three behind what a single-classed spellcaster or manifester would have. You aren’t really getting a bonus so much as catching up.
Thus, consider the effects of the 3.5 feats Practiced Spellcaster or Practiced Manifester: you gain a bonus to Caster Level or Manifester Level that may not exceed your total level. In other words, they let you “make up” missed levels, but don’t let you go ahead of where you would have been otherwise. This is a far, far less powerful effect, and more accurately describes what this item would do for you.
So let’s impose that limitation on the item:
When you wear these bracers, you get a +4 bonus to your Caster Level and Manifester Level, but this bonus cannot allow your Caster Level or Manifester Level exceed your total Hit Dice. You may have different bonuses for different classes depending on how close each is to your total HD.
Thus, as a Wizard 3/Psion 3/Cerebremancer 5
11th level, in total
Caster Level 11
3 Wizard levels + 5 Cerebremancer levels + 3 from the item
If you got the full +4, you would go over your HD
Manifester Level 11 as well (same calculation but with Psion levels)
If you later level up several times so that you are a Wizard 3/Psion 3/Cerebremancer 10/Loremaster 4:
Now, what would this item be worth? Still quite a lot, but it’s drastically limited compared to the full +4 item. The bonus is akin to taking two quite-good feats, but it’s only really useful to a cerebremancer and for them it’s quite crucial, so I’d favor pretty lenient pricing. Personally, bonus squared times 3,000 gp would be sufficient, though that’s largely because I would want to make it relatively easy to catch up; depending on your campaign and DM, I could see that number go as high as 6,000 gp. Anyway, with my own number:
+4 bonus (capped at HD):
42 × ~3,000 gp = ~48,000 gp
+6 bonus (capped at HD):
62 × ~3,000 gp = ~108,000 gp
For the sake of completion, a +3 version is the highest you need until 17th level, so:
32 × ~3,000 gp = ~27,000 gp
Obviously, still extremely expensive items, but they are extremely valuable to you.
Make sure you remember exactly what this bonus does!
A bonus to Caster/Manifester Level improves the spells/powers you can already cast/manifest: greater range, larger area, longer duration, more damage, anything that references “level” in the spell or power’s description. For psionics only, an increase in Manifester Level also increases bonus Power Points for having a high ability score and increases the limit on how many power points may be spent when Augmenting a power.
The bonus does not, however, increase your general spellcasting/manifesting ability. It does not give you more base spells per day or base power points, it does not allow you to cast/manifest higher level spells/powers, and it does not let you gain additional spells/powers known. An item that did that, even for a single level, even limited by your total HD, would be worth more than even a 20th-level character could afford. An item that did that should never be allowed into the game.
Best Answer
You asked for opinions, so in your position I'd get a wand…
The 4th-level sanctified spell greater luminous armor [abjur] (Book of Exalted Deeds 102) is one of the game's premier defenses. Not only does it provide a nearly unparalleled (albeit unenhanced) armor bonus to AC plus the daylight effect, but also the spell causes the affected creature's foes to suffer a −4 penalty on melee attack rolls against the affected creature, an effect that's largely unique in the vast 3.5 corpus. Further, because the character in question possesses the type undead, and because the DM has apparently agreed that the character can still cast sanctified spells despite an immunity to physical ability damage and drain, the character can cast the greater luminous armor spell with impunity.
So rather than create a new magic item that incorporates a greater luminous armor effect, I'd buy or create a wand of greater luminous armor (4th-level spell at caster level 7) (21,000 gp; 0 lbs.). One charge lasts a decent 7 hours, and the armor spell is dismissible so deactivating it—therefore the accompanying daylight effect—isn't an issue. Just don't be afraid to actually use that wand.
That 21,000 gp price tag may seem steep, but it's not. My back-of-the-envelope calculations using Table 7–33: Estimating Magic Item Gold Piece Values (Dungeon Master's Guide 285) puts the price of a new magic item that creates a continuous (or continuous-ish) greater luminous armor effect at over 40,000 gp. However, even a reasonable DM could require that a new magic item separate the greater luminous armor effect into pieces and price them individually. Adding the price of bracers of armor +8 (DMG 250) (64,000 gp; 1 lb.) to a custom at-will daylight item to a minor cloak of displacement (DMG 253) (24,000 gp; 1 lb.) (approximately equivalent to the penalty the armor inflicts), for instance, runs over 100,000 gp.
So if it's AC you want, instead of spending 100,000 gp on a new robe of light (or whatever you'd call your new magic item that creates the greater luminous armor effect), buy the wand. Then head to the Magic Item Compendium and its Table 6–11: Adding/Improving Common Item Effects (234). Slap onto your robe a +1 deflection bonus to AC or a +1 enhancement bonus to natural armor so that it's now a robe with a market price of at least 2,000 gp. Then, as levels are gained, bling out your robe further according to Improving Magic Items (MIC 233).
Anyway, then take that robe of protection +1 (or whatever) for the feat Item Familiar (Unearthed Arcana 170). If you must.
…And I'd skip the Item Familiar feat
If the Item Familiar feat were free, my PCs would never invest life energy, skill points, or spell slots into the item familiar. The risk of losing the item is simply too great. Even if the DM says that PCs' stuff won't be targeted, the prospect of an item that occupies a body slot being accidentally destroyed or lost is very real given the vagaries of the adventurer's lifestyle.
More practically, a PC who has invested life energy into the item gains XP at a different rate from PCs without the feat. This may not bother other PCs, but it makes adventure design harder for the DM, therefore tacitly encouraging the DM to destroy the item (because DMing is already hard enough). However, even if events are reasonable or its loss is accidental, if the item is lost, the effects are so severe that the DM looks like a jerk because the player opted to have the PC invest life energy (and maybe more) into the item. Once the PC invests in the item, it's lose-lose for the DM. And that's with a conscientious DM. If the DM's playstyle is adversarial, then I'd expect to lose the item regularly.
If the goal is not to invest in the item familiar and, instead, just have a magic item that grows in power with the character, consider the feat Ancestral Relic (BE 39, 41).
Note: The player rolled a natural 1 on his PC's Reflex saving throw against a spell that dealt sufficient damage to destroy the PC's item familiar. After realizing how terrible that was, I banned the feat in later campaigns; it simply provoked too much anxiety. The only time I'd consider the feat Item Familiar is if the campaign mandated that PCs all be truenamers from Tome of Magic (191+). If that's the case here, please point me to that campaign's online journal.