I've never really liked having these sorts of items in the game. By the time you're level 13+ you have hundreds of thousands of GP in assets. Buying a dozen +10 items for limited use is just too cheap. Instead of using skill points to get a good Craft skill, just buy a +10 item for the skill you want! Why bother with Rogue levels when you can just get items for Hide and Move Silently?
Personally I'd do it this way:
1> Increase the price by a factor of 2.5, to 50*(ranks^2). You could vary it by skill, of course. Double if it's a slotless item, as usual.
2> Require equal ranks instead of half; you can't put knowledge into an item that you didn't have yourself.
3> The bonus acts as ranks of the skill for the purposes of skill checks. The skill is treated as being a class skill for this purpose, but the bonus cannot take you over the maximum class-skill rank for your level. So, if the total of your own skill ranks plus those from the item exceed (level+3), the excess is lost.
4> The benefit only applies if you've been wearing the item for 24 hours. (Use the attunement rules from a Ring of Sustenance as a guideline). If you want an item with no attunement time, double the cost again.
Example: a level 4 Wizard picks up a ring that adds 10 to Perform (cost: 5,000 gp). At that level his maximum rank is 7, so for the purposes of Perform checks it'll be as if he had 7 ranks in the skill. Effectively these become useless to anyone who already had maxxed out a class skill.