Existing skill items are:
1> Too cheap.
2> Too easy for people to raise cross-class skills, covering inherent gaps in their character class.
3> Too easy to raise class skills ridiculously high, while Feats that add only 2 or 3 points are "balanced".
4> Too easy to swap off and on. If you suddenly need Hide skill, pull that Cloak of Elvenkind out of the haversack, and instaRogue!
Since this is a House Rule forum post, here's what my friends and I came up with a while back, although we haven't implemented it IMC. (Yet.)
DMG says 20*(bonus^2) gp. Change it to 100*(bonus)*(caster level) gp for the powerful skills, half as much for less useful skills. It's a DM's call, but the only things that I'd put in the low-cost group are things like Jump or individual Knowledge/Craft/Profession skills. Slotless items are double, of course.
The "bonus" is treated as skill points spent; that is, you get N skill ranks if it's a class skill for any of your classes, N/2 if it's cross-class. These pseudoranks are not actual ranks, so they don't violate the (3+level) maximum or let you qualify for PrCs. Likewise, Bards don't get new musical abilities from Perform items.
But, there's a new limitation: the item can't take your total rank beyond a number equal to its caster level.
(We're also considering an "attunement time" limit, where you have to wear the item at least an hour before gaining the benefit)
Let's say I make a Ring of Jumping that has bonus=10, caster level=16. Cost is 8,000gp since this skill is clearly in the "less useful" group.
If a Wizard (no ranks and cross-class) puts it on, he gets a +5 bonus.
If a Monk (no ranks but class skill) puts it on, he gets a +10 bonus.
If a Monk who already has 10 ranks in Jump puts it on, he'll hit the cap at +16 (10 from his own skill, 6 from the ring) since the ring can't take him beyond 16 ranks (its caster level).
(For 8000gp, the DMG says I could have a +20 ring)
What this does is kill those +30 items, since you'd have to be at least caster level 30 to make them. And, you stop having people take cross-class skill items so much. When the Cleric takes a Third Eye to get Spot and Search better than the Ranger has, there's a problem.
There'll also be some variety; If I want boots that raise Move Silently, for 5,000gp I could either get (+10 at CL 5, great for those cross-class people since they get +5) or (+5 at CL 10, great for Rogues who need a few more ranks), but not both.