I have a multi-part question here because as I understand the feat right now, it's awesome given the income to make it work for you. I've searched the forums but haven't found answers that I think are direct enough to quell my concerns. So let's get started!
First, it says you can inscribe a tattoo into a slot, but without using that slot. Does this mean that, for example, a character could have a cloak of displacement tattooed under a cloak of resistance and gain the benefits of both? Making sure just because that sounds "too good to be true" kinds of useful.
Second, pricing. I'm reading this as Wondrous Items are priced normally because it says "Magical tattoos follow the rules for magic item creation as though they were wondrous items, except that they can use the Craft (calligraphy, paintings, tattoos) skill." The only exception listed is the skill used, not pricing. Increasing the price to slot-less seems to be more focused on making sure you don't try to slot tattoos into anything but the locations already designated in the feat text.
The feat in question
And in closing. Was anyone around in 3.5 to remember Bunko's Bargain Basement? Is there a Pathfinder equivalent somewhere?