My coworker and I were discussing noble rooms, and got onto wondering what would make them happy.
Poking around on the wiki, we came up with the rent divisions for each quality level.
Which then bore the question: what factors into the cost of rent.
So I went about doing a lot of tests, and got the exact numbers that contribute to rent.
First off, I'll go into my methods, so people don't ask "how'd you get these numbers?"
(Note that in every case where rent was being checked, the room was freed, and then re-designated, as the rent only appeared to update at that point - I'm not sure if this is because the game normally only makes this check periodically, or not. (Care to comment Toady?))
I started out with a 1x1 room, in grey rock. In essence, a 1x2 hallway, with a door, and a bed. The cost of the furniture was already known to be a direct cost, and by swapping out the door, I realized the door is simply another piece of furniture as far as the rent is concerned. That left walls, and floor, with an unaccounted rent factor of 5. There are three wall spaces, and 2 floor (one under bed, one under door).
Rock walls, and rock floors, appear to be 1 each. I tested this by checking the rent of some simple rooms, and it added up. I then proceeded to smooth a wall, and a floor, checked some other rooms, and these numbers added up too. I then expanded these tests to cases which had light/dark stone, obsidian, and gems, and obtained an exact value that worked for them all.
Then poking around, I discovered the material table on the wiki... and the numbers matched. /sigh, so I wasted a couple hours, but at least I was on the right track. I still learned the values for smoothing, which were not in there, and varies by material type.
So, in a nutshell, each tile in a room counts for exactly the following:
Floors:
rough rock 1
smooth rock 4
rough light/dark 2
smooth light/dark 14
rough obsidian 3
smooth obsidian 21
Walls:
rough rock 1
smooth rock 5
rough light/dark 2
smooth light/dark 18
rough obsidian 3
smooth obsidian 27
malachite 2
casserite 2
sphalerite 2
coal 2
turqouise 2
Rock Crystal 2
Rose Quartz 3
aventurine 3
Galena 10
hematite 10
aquamarine 20
chrysoberyl 20
red spinel 20
opal 20
gold 30
platinum 40
diamond 40
emerald 40
ruby 40
sapphire 40
Now, you may notice that detailing is left off my list. That is because the results I was getting was random. I see a gain of anywhere from 0 to 50 on top of what is normally present for smoothed rock. It seems to be tied to the skill level of the engraver - someone with no skill would usually give me 0 or 10, while a legendary would normally give me 0, 40, or 50. Masterpieces were not excempt from this randomness either - they would sometimes result in a 0 gain too, which makes me think this is a bug. (Toady?)
Another thing to note is that statues behave in an odd manner, as do windows.
A 3x3 room had a rent value of 83. I took out one of the rock walls (worth 1), and put in a green glass window (worth 50). Resulting rent: 83.
Likewise, a 3x3 room entirely in rough rock, with a statue worth 250 had a rent increase of 250 when it was placed in the center, and a rent gain of 242 (less whatever was blocked by statue, since statues count as walls, which was 1 rock wall on side walls, and 2 rock walls in corners).
Due to the 250 gain in the center, it appears that squares with a statue do count their floor square still.
I'm not sure if the 242 gain instead of the full 250 is intended, or is a bug, likewise, I'm not sure if windows should be ignored in rent either (Toady?)