What it isn't.
Tattoo magic isn't balanced.
What it is.
It is ridiculous.
What is does.
Tattoo magic is meant to empower your DM driven NPCs with strange and mystical abilities akin to Psionics, another broken powerful set of rules.
Tattoo magic effectively allows a dungeonmaster to graft whatever abilities to whatever NPC. Much like psionics. You just pick and choose stuff. That your character can do, from a massive range of monster like abilities.
For example: One of my characters, has tattooed the magicks for misty step to their feet, so even though they aren't a magic using character, they can in fact misty step.
What's it cost?
I don't care about specifics, but let's rough it to 10,000 gold per level for the materials.
Let's write some rules.
And let's keep them nice and simple.
Ink has to be made of some stuff that's worth [Spell Level] x 1000 platinum (that's 10,000 gold pieces).
Ink takes 1 week per level to mix for a 1 inch by 1 inch tattoo.
Mixing ink requires the feat below.
Ink Mixer
You know how to blend inks used for magical tattoos.
You may mix ink for spells up to a maximum level equal to three times the number of times you've taken this feat.
Eg: If you've taken the feat 3 times, you can mix up to level 9 magical inks.
Drinking inks requires the drinker to roll on the wild magic table.
Creating the tattoo requires the feat below.
Arcane Tattooist
You are a skilled tattooist with a deep interest and understanding of the arcane arts.
You have learned to read and cast magic from scrolls regardless of class.
Using a tattooists needle, you can use magical inks to create arcanic effects upon a creature.
You may create a tattoo for any spell up to a maximum level equal to three times the number of times you've taken this feat.
Eg: If you've taken the feat 3 times, you can create a tattoo up to level 9 with the appropriate magical ink.
Creating the tattoo requires the tattooist translate the spell to a tattoo form.
Using an Arcane check with DC equal to 10 + the spells level, the tattooist draws the spell as a pattern of signs and sigils.
The tattoo takes a number of hours equal to [Spell Level] x [Number of Charges] to apply if it is maintaining its original duration.
The tattoo takes a number of hours equal to [Spell Level] x 8d6 to apply if it is to be a permanent effect.
The tattoo is 6 square inches for each level if it is maintaining its original duration.
The tattoo is 24 square inches for each level if it is a permanent effect.
Eg: A square inch is a postage stamp. 6 square inches would be a standard business card (3.5" x 2")
You may spread the tattoo across multiple body parts, including separate limbs.
Eg: A permament misty step tattoo upon the bottoms of the feet.