There are a lot of house rules for these. If you just want to play with costs a bit, I'd go (bonus^2) * 50gp for limited-use skills like Jump, and (bonus^2) * 100gp for more useful stuff like Tumble or Hide. Yes, that's 5 times the DMG list price, but to me it's still worth that much. On an opposed roll, a simple +4 modifier changes it from a 47.5% win, 5% tie, 47.5% lose distribution to 66% win, 4% tie, 30% lose one. This can be huge in practice.
But even that isn't enough, IMO. It's just too easy for someone to pick up a cross-class +10 skill item and be great. If you're someone who's already raised that skill, it makes you almost untouchable, but the fact that it levels the playing field too much hurts skill-based classes (like Rogues) more than anything.
Then, there are flukes for the set-DC skills. Put a +10 Concentration item on a caster and now he can cast everything defensively. Same goes for Tumble.
The cost structure I started using IMC was:
25 gp * bonus * caster level for limited-use or non-opposed skills (Jump, any Knowledge skill, any Craft, any Profession, etc.), and
50 gp * bonus * caster level for useful/combat skills or anything requiring an opposed check.
Then, change the rules like this:
> The "bonus" is "virtual skill points". That is, they give virtual skill ranks, except that the bonus is cut in half if it's not a class skill for any of your classes. These virtual ranks only matter for skill checks; you can't use virtual skill ranks to give more Bardic Music abilities (Perform) or qualify for PrCs.
> The item cannot give you a higher skill rank total (actual skill ranks + virtual ones from the item) than the item's caster level. It CAN give you more than your normal maximum of (level+3).
So, the Boots of Elvenkind might be +10 Move Silently items with caster level 12, for a cost of 6000gp.
> If you're a Rogue who already has a +12 or higher skill modifier, the boots are useless. On the other hand, have one made that's +5 with caster level 20 (same cost), and you're fine.
> If you're a Rogue who hasn't spent any points on Move Silently, you'll probably get the full +10 bonus. But, if you start raising the skill on your own, you'll get less and less use out of them, until eventually you're so skilled that they're not helping any.
> If you're a Wizard, you'll get a +5 bonus.
Anyway, the numbers can be tweaked as needed.
EDIT: Forgot to add, we got the idea from the cybernetic chip implant rules from games like Shadowrun or Cyberpunk. That is, the chip gave you skills by letting you mimic the recorded actions of someone who knew what they were doing. You'd be able to actually DO the stuff they did, whether it was martial arts or auto repair, but you wouldn't really understand why it was working; once you removed the chip you wouldn't have gained any actual skills. And, if you already knew how to do it, the chip wouldn't help.
So, we just said that the magical +skill items were the same sort of thing