I always thought depreciation or rise in value for art objects is random.
I don't know about art objects by other modders, but I have Merola's Multi-painting and sometimes it sells for a few dollars above the purchase price of $399. Maxis items don't always depreciate too. As for paintings done by sims, I know it's possible for them to depreciate in value if the finished paintings had been left on the easel for some time.