*Fiddle, fiddle*
Well, looking at my existing Room Value Calculation sheet, a 1x1 room with a diamond encrusted bed, no beneficial value from height (it would be minor, due to the increased number of walls and maybe a little more), and no door then the room would be worth....
604.06 DwarfBux. Vanilla room value is 7329, assuming rough gray rock walls.
That's Decent Quarters vs. Grand Quarters. Just smoothing the room adds 34 raw vanilla value, but increases my calculation for the room bonus enough to push the value up to 1779.9 "Great Quarters."
However, even decorating the bed in worked adamantine doesn't push the room value into royal. You'd need to have built the room in "white" or "black" stone and smoothed it. My original post on the topic included various examples and how much work it took to get a room up to "royal" without using ungodly expensive materials (diamonds and adamantine, due to their rarity, I think I stuck with gold). IIRC it involved at least one engraving (any quality) in black stone at 1x1 size with two pieces of masterwork platinum furniture one of which is decorated masterfully in gold (10468.76 "averaged" value).
Anyway the point is, that even with a bonus from having vaulted ceilings, 90% of a room's value is from furniture. My (complex, but reasonable) formula simply puts more weight on the room itself.