> Who is more upper class, a relatively impoverished Prince of the Holy Roman Empire or a billionaire by marriage and luck?
I think Western society has been undergoing a transition between two different class systems. (Marxists would surely connect this with the economic transitions from feudalism to mercantilism to early capitalism to late capitalism.)
In the mediaeval class system, class was determined by social status which was predominantly inherited, and money alone was not enough to move from the bottom to the top. A person born poor might acquire great wealth, but they would still be locked out of the nobility/aristocracy, unless the monarch deigned to ennoble them. An impoverished baron socially outranked the far wealthier merchant who was born into poverty and worked/lucked their way out of it.
In the late capitalist class system–all that really matters is your net worth. Everything else is secondary. Who cares who your parents were or where you come from once you have a billion dollars to your name – and if a billionaire is excluded socially, it is very likely due to something about their individual behaviour (see e.g Kanye–although I suppose he's an ex-billionaire now), rather than their family background in itself.
The transition between the two systems has been ongoing; it still hasn't completely finished, but we've moved a lot closer to the late capitalist system, and a lot further away from the mediaeval system, than we were 50 or 100 years ago. I don't think the transition proceeds at the same rate in every country either – I think the UK retains more of the old class system than most other places (although even in the UK it is a lot weaker than it used to be); it also arguably retains some strength in the northeastern US, although not to the same extent as the UK.