Earlier quoted context omitted.
To answer that you need a way of equating money with "utility" (i.e. the use and/or pleasure that people get from their money). I believe that utility is logarithmic with rate of expenditure. Hence doubling your spending rate gets you a constant increment in utility. This is why we measure pay rises in percent rather than absolute value. Another $100/week would make a huge difference to someone on the breadline, but…
My guess is it would be more of a logistic curve (S-shaped) rather than logarithmic. $1 does not make you much happier than 1¢, but $50k/year is a big step up from $30k/year. At the low end, at some point you can afford to have a roof over your head, and an extra $20 might mean that you're eating much better for a whole week, or you can buy new shoes, or whatever. But if you're homeless, $20 won't feed you for very l…
A few years ago, I met a beggar at a bus stop, who said that the store across the street had a sleeping bag for 50% off until the end of the week, and he was hoping that he would be able to buy it for the winter. I didn't have much money on me myself (basically just the bus fare), but I gave him what I had and walked home instead. To him, those five-ish euros meant not freezing in his sleep, while to me, they only meant a bit of unplanned exercise.
If you have nothing at all, every little bit of money gives you access to some tangible improvement of your situation. But if you have much less than nothing, and every cent of additional income just serves to lower a gigantic debt, there isn't really much incentive to earn more when it makes no significant difference in the end. This is probably one of the reasons bankruptcy law allows discharging unpayable debts, because otherwise debtors might not be motivated to do something about the debts they could repay.