I'm playing a promethean alchemist i want to add toughened hide to my humoculous.
Toughened Hide: By adding diamond dust and cold iron to the homunculus, a crafter can increase its natural armor bonus to AC by 1, 2, or 3. Price: +1,000 gp (+1), +4,000 gp (+2), or +9,000
A promethean alchemist humoculous already has 2 natural armor, does the wording allow me to get a 15 or do I use 9000 gold to get 13?