There is a specific property of the headband which interests me.
A headband of vast intelligence has one skill associated with it per +2 bonus it grants. After being worn for 24 hours, the headband grants a number of skill ranks in those skills equal to the wearer's total Hit Dice. These ranks do not stack with the ranks a creature already possesses. These skills are chosen when the headband is created.
Now the lowest level of this item is 4,000gp, which would be 2,000gp to make yourself. To raise a skill by 5 points would be 2,500pm (or 1,250 self made), 6 points would be 3,600pm (1,800gp self made), and +7 would be 4,900gp (2,450pg self made). So once you reach level 7 the item starts paying itself off compared to an item which specifically increases the skill by a fixed amount. The numbers for the skill points increase comes from the chart in creating magic items. Skill bonus (competence) Bonus squared x 100 gp
I had the thought of stacking multiple headbands together as a single item and was wondering, specifically how this would work, and ways to keep the cost down. So first off, you would not get stacking int bonuses, there would be the +2 and no more. What about the skills however? Since each "headband" applies to a different skill would each skill be always active with the bonus? Would you be required to toggle between which skill is getting the bonus from headbands you are wearing (if so then what kind of action would be required?)? The most logical way would likely to be that you have multiple headbands and you switch them out as needed, except that for the skill ranks, you must wait 24 hours.
In the magic item creation rules there is a small part about similar abilities costing less, and different abilities costing more.
Multiple Similar Abilities: For items with multiple similar abilities that don't take up space on a character's body, use the following formula: Calculate the price of the single most costly ability, then add 75% of the value of the next most costly ability, plus 1/2 the value of any other abilities.
Multiple Different Abilities: Abilities such as an attack roll bonus or saving throw bonus and a spell-like function are not similar, and their values are simply added together to determine the cost. For items that take up a space on a character's body, each additional power not only has no discount but instead has a 50% increase in price.
So, these rules are only for slotless items, which can still work. So a headband costs 4000 and slotless makes its 8000, adding on a second one would be 6000 for a total of 14,000, and then 50% for each one after that, at 4000 each.
Mostly I was thinking of this kind of item to support characters who have very low amount of skills per level but are willing to spend the gold to take care of it. But I have an issue with the golden rule of magical item creation
The correct way to price an item is by comparing its abilities to similar items (see Magic Item Gold Piece Values), and only if there are no similar items should you use the pricing formulas to determine an approximate price for the item. If you discover a loophole that allows an item to have an ability for a much lower price than is given for a comparable item, the GM should require using the price of the item, as that is the standard cost for such an effect. Most of these loopholes stem from trying to get unlimited uses per day of a spell effect from the "command word" or "use-activated or continuous" lines of Table: Estimating Magic Item Gold Piece Values.
Because at some point, the headband becomes far more potent than the skill enhancement, but at a much lower cost.