How much do you tip a tattoo artist?

say you got something relatively small that was only about $120....what would you tip them? what about if you got something big around $4-500 what is a reasonable tip then? I think I'm generally a good tipper...pizza always get $5 if i'm just getting like a single large and at bars its $1 per drink then $10 at the end of the night but I have no idea on tipping tattoo artists.