The liberal think tank Economic Policy Institute (EPI) has developed a Family Budget Calculator that tallies the annual income needed to support a family of four to cover the cost of seven needs: transportation, food, housing, child care, taxes, and "other necessities."
Here is a chart from the Washington Post comparing the 10 largest US metro areas by population (including Washington, DC - the home of hundreds of think tanks) showing the annual cost of living.
The average annual cost for a family of four to live in Washington, DC - the home of 396 think tanks and counting - is $106,493. The average think tank salary is somewhere between $47,000 and $66,000.
It shows that in Washington, DC, if you work at a think tank and are the sole breadwinner with a family, you basically will not be able to live in Washington (unless you are at the upper echelons). Happy think tanking!