The "what someone will pay for it" definition is a trojan horse. It looks like a mathematical practicality, but it actually smuggles the self-interest of the wealthy into a value system that would not otherwise enshrine such as a guiding principle.
Moreover, there is a difference between economic and social value, which the field of economics already recognizes in the form of positive externalities.
Rich people who mostly want others to pay them for being rich? That's a weird and extreme answer and I'm tired of pretending it isn't. This is not an externality problem, it's a priority problem. These priorities are dogshit.
We don't need gini=0 to fix this. Gini=0.9 is nuts, famous billionaires having millions of times the weight as the median American is nuts, and we need to back off a bit. Maybe start by asking them to pay taxes. For once.
As wealth inequality increases, the purpose of the economy as seen through its objective function increasingly revolves around the desire of rich people to get paid for being rich. The exponential growth of asset values in excess of economic growth (including yours, but dominated by those of the wealthy) ensures that not only does the growing economic surplus fail to trickle down, but that the median person sees a grinding deprivation as their ability to command a wage is slowly eroded while their costs rise.