"Foster parents should receive the funds they need," says co-author Julie Farber, director of policy at Children's Rights, a New York-based advocacy group. Too often, she says, they either stop taking in foster kids or dig into their own pockets to pay for prom dresses and Boy Scout uniforms.

The report comes as more states report a shortage of families to care for the 500,000 children nationwide in foster care.

The report estimates the national average for monthly costs for healthy foster kids at $629 for a 2-year-old, $721 for a 9-year-old and $790 for a 16-year-old.

The report, citing the wide range of state payments, recommends a uniform rate that can be adjusted to account for varying costs of living. Its proposed rate is based on expenses reported by middle-income families in a Labor Department survey, plus a 22% increase for extra food and other things needed by foster kids who may have been malnourished or abused.