BlueJay91,
I saw this question early yesterday morning, and thought I'd do a bit of testing.
For paintings I believe it's a flat $10 per night at midnight ...
Metro's dead-on, from what I can see, "Large Paintings" produced by a Level 10 Painter seem to increase in value $10/day (regardless of whether they are in your Sim's inventory, on the easel, on a wall, or a basement wall ... or what the painting's value was when it was created). I'll post the results once the paintings "age" to two-weeks (and my low-level 4 Painter produces a few "large" paintings too).
I think your $118 painting was produced by a Level 5-6 Painter? I think the Painter's skill level seems to have something to do with it's increase, but I'm not sure why it wouldn't increase in value over a Sim Week or two (other than the ORIGINAL value was low enough that even the small percentage daily increases are too small to see).
Hope this helps, and I'll post my charts once 2 Sim Weeks have passed.
-- Happytime