First of all,sorry for the necro. I thought of a house rule solution to my art problem, and felt I should bring it here.
You can have a Performance check for the painting, with gradual steps in success or failure. Failure by 5 or more can lead to it being worth a quarter of the market price, while failing by less than 5 gives you half market value. Hitting the DC gives standard market value, succeeding by less than 5 gives double the value, and succeeding by more than 5 gives quadruple the market value. You could then modify this as the situation requires, depending on the rarity or picturesque qualities of the subject. For example, the painting of the Dragon council could have a base worth of 500 times the market value, based on Rarity of the sight, and the inherent beauty of the Dragons. Failing the DC by 5 or more would only get you 125 times the market value. However, succeeding by more than 5 would get you 2000 times the market value. Of course, you could just go for the basic value of 500 times, instead of rolling. On top of this, a charismatic PC could then try to bargain the price even higher during the sale, maybe pushing three or four thousand times the market value for a painting.
Any thoughts?