To show wealth in sci-fi, I often see media use expensive materials on Earth like gold to decorate thrones and throne rooms. I think this shorthand makes sense, but is misguided.
I think the rarest thing for a spaceship to have on board are materials like synthetic rubber and plastic, and I don't see much sci-fi mentioning it.
Update: thanks to @colin introducing me to how the methane to petroleum pipeline of "natural gas" works, it sounds like bacterial cultures could allow creation of petroleum (and therefore rubber and plastic) in space.
With that in mind, the runner up seems to be single-slab redwood: it's hard to grow because it needs lots of space and time, it's hard to synthesize trees that look legitimate, and single-slab means you can't grow smaller trees faster and then combine them
Empty space.
Space requires support, whether it's life support systems or just the building around it. It takes away from what could be useful- even necessary- structure. It requires upkeep like anything else, even while it doesn't serve any purpose.
Empty space is wasted space, and what more clearly signifies great wealth more than "I have this valuable thing, and can knowingly piss it away"?
