There are base gold costs for constant skill-granting items (http://www.d20srd.org/srd/magicItems/creatingMagicItems.htm), but I cannot find anywhere what the caster level requirements should be for such items. There are no high-level +skill items without other craziness that I can find, so it is difficult to judge what the progression should be. Is there a guideline anywhere for this issue?
e.g. slippers of tumbling +5/+10/+15/+20/+25/+30 competence bonus to tumbling
Skill items don't have caster level requirements. Just like most magic items don't have caster level requirements, other that "whatever it takes to cast the spells involved". (cite)
The DMG guidelines for custom items don't say how to choose crafting requirements at all, let alone specifically for skill items.
Most existing skill items' crafting requirements fall into one of two categories, with no apparent rule to predict which. They even often come in both versions for the same skill:
Therefore, you should be able to craft a +30 Slippers of Tumbling at character level 3, if you somehow had enough money for it. But if your DM is being stingy, you might need character level 27 so that you can have 30 skill ranks in tumble. The crafting requirement definitely won't be anywhere between those two.
Existing skill items, sorted by type of crafting requirement, from DMG and MIC (I haven't looked through other books for examples):
Shadow armor, +5 hide (DMG): invisibility (Sor/Wiz2)
Greater Shadow armor, +15 hide (DMG): invisibility (Sor/Wiz2)
Silent Moves armor, +5 move silently (DMG): silence (Clr2)
Greater Silent Moves armor, +15 move silently (DMG): silence (Clr2)
Slick armor, +5 escape artist (DMG): grease (Sor/Wiz1)
Greater Slick armor, +15 escape artist (DMG): grease (Sor/Wiz1)
Balance armor, +5 balance (MIC): balancing lorecall (Sor/Wiz2)
Greater Balance armor, +15 balance (MIC): balancing lorecall (Sor/Wiz2)
Ring of Chameleon Power, +10 hide (DMG): invisibility (Sor/Wiz2)
Robe of Blending, +10 hide (DMG): disguise self (Sor/Wiz1)
Circlet of Persuasion, +3 to all Cha-based skills (DMG): eagle's splendor (Sor/Wiz2)
Eyes of the Eagle, +5 spot (DMG): clairvoyance (Sor/Wiz3)
Gloves of Swimming and Climbing, +5 swim and climb (DMG): bull's strength (Sor/Wiz2), cat's grace (Sor/Wiz2)
Salve of Slipperiness, +20 escape artist (DMG): grease (Sor/Wiz1)
Vest of Escape, +4 open lock, +6 escape artist (DMG): knock (Sor/Wiz2), grease (Sor/Wiz1)
Boots of Jumping, +20 jump (MIC): jump (Sor/Wiz1)
Monocle of Perusal, +5 appraise (MIC): identify (Sor/Wiz1)
Spellsight Spectacles, +5 umd and spellcraft (MIC): read magic (Sor/Wiz0)
Tunic of Steady Spellcasting, +5 concentration (MIC): bear's endurance (Sor/Wiz2)
Ring of Climbing, +5 climb (DMG): 5 ranks in climb
Ring of Improved Climbing, +10 climb (DMG): 10 ranks in climb
Ring of Jumping, +5 jump (DMG): 5 ranks in jump
Ring of Improved Jumping, +10 jump (DMG): 10 ranks in jump
Ring of Swimming, +5 swim (DMG): 5 ranks in swim
Ring of Improved Swimming, +10 swim (DMG): 10 ranks in swim
Boots of Striding and Springing, +5 jump (DMG): 5 ranks in jump
Crystal Mask of Discernment, +10 sense motive (MIC): 10 ranks in sense motive
Crystal Mask of Dread, +10 intimidate (MIC): 10 ranks in intimidate
Crystal Mask of Insight, +9 search and spot (MIC): 9 ranks in search and spot
Crystal Mask of Knowledge, +5 to any one knowledge skill chosen at crafting time (MIC): 5 ranks in the same knowledge skill
Crystal Mask of Psionic Craft, +10 psicraft (MIC): 10 ranks in psicraft
Ring of Self-Sufficiency, +10 autohypnosis (MIC): 10 ranks in autohypnosis
Third Eye Concentrate, +10 concentration (MIC): 10 ranks in concentration
Skin of Nimbleness, +10 tumble (MIC): 10 ranks in tumble
Boots of Elvenkind, +5 move silently (DMG): creator must be an elf
Cloak of Elvenkind, +5 hide (DMG): creator must be an elf
Third Eye Improvisation, +5 to any one untrained skill chosen at use time (MIC): no prereq
As topquark mentioned, there is no caster level requirement for skills in either the DMG or MiC, additionally a look through EL(Epic Level Handbook) shows no requirements or limits either. The epic level handbook only gives one example of skill bonus, but its pricing follows that given by the DMG, even if it has a CL of 20th.
What does limit PCs acquisition of high level skill granting magic items is the exponential growth of costs. A single magic item with skill bonus costs: $$ Bonus^2*100gp $$ That combined with the DMG recommended maximum value per item:
...you can limit him to owning no single item worth more than one-quarter of that[maximum wealth]... (PG199 DMG3.5)
Breakdown of bonus and minimum level achievable vs DMG recommended Level
Bonus Cost MinLeVeL(Wealth%) Recommended(<25%)
+5 2,500gp 3rd(93%) 6th
+10 10,000gp 6th(77%) 10th
+15 22,500gp 8th(83%) 12th/13th
+20 40,000gp 10th(82%) 15th
+30 90,000gp 13th(82%) 18th
+40 160,000gp 15th(80%) 20th
The relative power excessive skill bonus grant is much lower as compared to other items. I.e. a +30 ring is 90k, a ring of three wishes is 97k.
Final note, since skill bonuses are competence, no stacking. Alternate possible bonuses that would stack(like luck) are about 10-20 times more expensive.
