10,000 gp to buy a +10 competence bonus to a skill check. Half that if you craft it.
Wieldskill wasn't mentioned some how? It's like core.
Guidance: various 0, +1 competence bonus
Improvisation: Bard 1 (Spell Compendium), up to 1/2 CL luck bonus
Focusing Chant: Bard 1 (Spell Compendium), +1 circumstance bonus
Divine Insight: Cleric/Paladin 2 (Spell Compendium), 5 + CL (max +15) insight bonus
Guidance of the Avatar: Cleric 2 (WotC website, Spellbook Archive), +20 competence bonus
Recitation: Cleric 4 (WotC Website, Spellbook Archive), +2 luck bonus
Crown of Glory: Glory 8 (Spell Compendium), +4 morale bonus
Edit: You can actually craft a skill item granting up to a +30 enhancement
bonus without being an epic item. The +30 limit is explicitly stated for enhancement bonuses to skills, but there is no such similar wording for competence
bonuses to skill checks so you can have as much of a competence bonus to a skill via a magic item as you want so long as the total market price runs under 200k.
These limitations, and lack of limitation on the latter, are spelled out here
In the end, technically it'd cost you only 15k gp to craft a +30 skill bonus item and never need to roll UMD again.