| Gremlington |
Pathfinder Unchained actually added a section of skills that might help you out here! The Background Skills added two new skills, which were Lore and more importantly for you: Artistry. Artistry functions very similarly to Craft, and it includes a table with DCs for crafting various works as well as commission prices for them.
And I agree with the line of thought John Napier 698 was on, in terms of adding a masterwork component for truly spectacular pieces. Perhaps for very important life's work kinds of art?
Hopefully this will at least point you in the right direction!