There are more than 4,000 children in foster care in Scotland
All foster carers in Scotland should be paid a national minimum allowance, a charity has claimed.
Fostering Network Scotland said many carers were left out of pocket by a "postcode lottery" on allowances.
National minimum allowances have already been set up in England and Northern Ireland, but in Scotland the payments are still set by councils.
The Scottish government said it believed local decisions on allowances were better.
Fostering Network Scotland said its research suggested that it cost foster carers at least £125 a week to look after a child under the age of four.
This rose to £216 for a child between 16 and 18, the charity said.
It found that half of local authorities in Scotland had paid less than the charity's recommended rates last year, meaning many foster carers were left with no choice but to either dip into their own pockets, or see their fostered children go without.
Fostering Network Scotland director Sara Lurie called on the Scottish government to show it fully values children in care by ensuring all foster carers across the country have enough money to pay for the things that their fostered children need.
"At a time when more children are coming into care and 1,700 more foster families are needed in Scotland, it is vital that the government addresses this as a matter of urgency," she said.
"How does it propose to recruit more foster carers to look after some of the most vulnerable children if it can't even guarantee that they will be able to make ends meet?
"Our survey showed half of local authorities do give their foster carers adequate allowances, so it is not an impossible task.
"But the only way to bring an end to this postcode lottery is to make payment of proper allowances a statutory requirement. Otherwise foster carers and fostered children will continue to lose out."
A Scottish government spokesman said: "Local authorities are best placed to make decisions about the levels of allowances.
"We support them in moving towards payments at the Fostering Network's recommended rate and are pleased that an increasing number of councils are doing so. However, these are decisions best made locally."
About 6,160 children are looked after away from home on any given day in Scotland.
Of these, 4,240 live with 3,200 foster families. The Fostering Network estimates there is a shortage of 1,700 foster families.