Found on d20srd.org (emphasis mine)
While not truly an artifact, the epic magic item is a creation of such power that it surpasses other magic items. Epic magic items are objects of great power and value. The following are typical characteristics of an epic magic item. In general, an item with even one of these characteristics is an epic magic item.
Grants a bonus on attacks or damage greater than +5.
Grants an enhancement bonus to armor higher than +5.
Has a special ability with a market price modifier greater than +5.
Grants an armor bonus of greater than +10 (not including magic armor’s enhancement bonus).
Grants a natural armor, deflection, or resistance bonus greater than +5.
Grants an enhancement bonus to an ability score greater than +6.
Grants an enhancement bonus on a skill check greater than +30.
Mimics a spell of an effective level higher than 9th.
Has a caster level above 20th.
Has a market price above 200,000 gp, not including material costs for armor or weapons, material component- or experience point-based costs, or additional value for intelligent items.
So it seems that anything +30 or lower is considered 'non-epic'. An item that gave you a +30 bonus to one skill would have a market value of 90,000 gp, by standard formula.
The limiting factors here (assuming your DM allows both the purchase of magic items, and the creation of 'non-book' items) would be the resources of the city you're in, and the wizard you find to create it. You'd have to be somewhere that could supply the 45,000 gp in raw materials, and you'd have to find a wizard willing to burn 3600 xp to do it.
Also, looking at most other skill-enhancing items in the DMG, the prerequisites to make then include having at least the same number of skill ranks as you are giving in magical bonus (ie, creator has to have at least 5 ranks in Climb for a Ring of Climbing, 10 ranks in Climb for a Ring of Improved Climbing, etc). If your DM goes with this trend, then you'd need a wizard with 30 ranks in Bluff to make a +30 item. This is, of course, only possible at epic levels. NOt only hard to find, but even if you do, with a Bluff that high, he'll talk you into paying a lot more than 90,000 for your item.
Of course, nobody's holding a hand crossbow to your head and forcing you to buy the +30 version. Most of the standard DMG items cap off at +10.