If you're poor, you spend a lot of your income on food

To me, one of the best measures of wealth is the percentage of one's income that is spent on food. If you check out this map, you'll see that the wealthy spend down around 10% of their income on food, whereas those in poverty spend up to 50%.

This is also a good way to intuitively grasp the dramatic increase in total worldwide wealth since the industrial revolution. I tried finding looking for some historical data for this but my Google-fu wasn't strong enough. If I find something though I'll post it.