Maybe something like median expected total accumulated net worth at longest known human lifespan? You'd want something that isn't distorted by income tails and reflects loss due to shortened lifespan.

It's fuzzy though and gets dystopian really quickly.

The easier method to do a more nuanced analysis is to use data visualization where you put income on one axis and lifespan on another. https://www.gapminder.org/answers/how-does-income-relate-to-...

Interesting question though. Someone's probably thought of this already and it's just not in wide use. Maybe something like annual income in dollars cut by average life expectancy?