It’s more complex than that.
Let’s do an experiment:
Remove one of players. Look at 30€/M you have in company wallet and try to say which player you removed (one from Europe, one from Russia or one from China?).
Impossible, isn’t it? 15€/M is 15€/M no matter where it came from.
Now you can say that you are talking about spending and not about paying. This makes sense but you should consider that people are different. Even in the same country you have very different wealth. So even for two players from China the same 15€/M is completely different values.
You will talk about average salaries. But you should understand that since EvE is not average entertainment then it would be wrong to expect EvE players be equally spread over population thus making averaged salaries bad approximation to purchasing power of actual and potential players.
(huh, wall of text!)