The income level against the COST of living in that country is the usual definition.
OK, that one makes actual sense.
But, at the same time, I had an economics teacher once point out that no matter what earnings are in any country, the cost of living in that country will balance out to coming out to about that level.
If the population only has x amount of money on rent on average, then that is what makes sense to charge. Then, there are those business minds who find poorer neighborhoods and charge lower rent there, of course offering lower services and quality of lodging.
Numbeo's numbers come out to (USA):
Rent 3 bedroom not in centre $1231
(Fun little note, in some cities and counties in the USA, if you have kids, you are required to have a seperate bedroom for them, and if they are a boy and a girl, must have seperate bedrooms as well, some cities even require that after a certain age same sex siblings cannot share bedrooms)
Utilities $200
Food $800
(According to USDA per week In June 2010, the thrifty plan for a family of 4 per week was $134.50. Low-cost = $175.10 Moderate cost = $219.10 Liberal cost = $265.90)
Often, if you rent you by law must carry renters insurance $10/mo
Add in car payment $300
Car insurance $100
Gas $500
(I'm ballparking based on what some others I know pay, I don't pay this amount myself)
Telephone and internet could be $100
Clothes $100 (family of 3-4)
Total per month $3331, per year $39972
It's just a educated guesstimate. But, basically, the poverty rate in the USA is pathetically low compared to cost of living in the USA. I believe the median cost of living is $51,914 quickfacts.census.gov/qfd/states/00000.html
So, can someone break down using those same above catagories the cost of living in Poland? We could compare that to the poverty level vs average income level in Poland to get an idea.