Crafting a wondrous item requires 1 day for each 1,000 gp of the base price.
Item Creation Feat Required: Craft Wondrous Item.
Skill Used In Creation: Spellcraft or an applicable Craft or Profession skill check.
Headband of INspired Wisdom from +4 to +6 = 10 days
The cost to add additional abilities to an item is the same as if the item was not magical, less the value of the original item. Thus, a +1 longsword can be made into a +2 vorpal longsword, with the cost to create it being equal to that of a +2 vorpal sword minus the cost of a +1 longsword.
(ex): Headband of Inspired Wisdom from +4 to +6 = 10,000gp
If the item is one that occupies a specific place on a character's body, the cost of adding any additional ability or increased bonus to that item increases by 50%. For example, if a character adds the power to confer invisibility to her ring of protection 2, the cost of adding this ability is the same as for creating aring of invisibility multiplied by 1.5.
(ex): Headband of Inspired Wisdom from +4 to +6 = 10,000gp x 1.5 = 15,000gp