Today I spent several happy hours working away on adding tattoos to my economic system. Last session my players, having arrived in a port city, were excited (among many other things) about the possibility of finding a sailor who could give them some. Since I recently programmed the recipes needed to produce paper, I was already mentally ready to spend time work researching ink, so a lot of the time today was spent looking into pigments, binders, dyes, and mordants, and looking for historical information on the production of same. A good chunk of that time was spent looking in vain for some example ratios of pigment, binder, and water for pigment-based inks; in the end, for the red/yellow (both ochre) and blue inks, I had to guess.
Afterward, having added black, red, yellow, and blue inks, I was ready to figure out how to price a tattoo. It was natural to decide to price by the square inch, and almost as easy to conclude on three difficulty categories for pricing: geometric (lines, shapes, and tribal patterns); lettering; and figure.
What was tricky was figuring out the volume of ink actually used up in the tattooing process. I looked far and wide, but the techniques of tattooing are too complex for there to be a simple "this many milliliters per square inch of skin" guideline. What I ended up doing was treating the "depth" of the square-inch unit tattoo as being one one-hundredth of an inch deep. This gave me a volume of ink used, which I could then convert to pints of ink. Shazam: my game has prices for three styles of tattoo in four colors (though I've just now realized that so far they are all single-color tattoos, and I should decide how I'm going to treat tattoos with multiple colors in them. Do I price them as multiple separate small ones? As one big one, and as if it used all four colors equally? Should multi-color tats have a higher difficulty? And so on. In practice, these questions are simple enough to answer logically at the table as necessary.)
Now, my players can design an image to their heart's content (or scare up something cool from the web), measure out its area, and be rewarded by having it added to their characters' bodies. The more exacting they are as a real-life designer, and the more spendy they get in-game, the fancier the tattoo they can have.
Or to put it another way ... tattoos are a new kind of goal that players can aim for in my world.