The number of children in council care in England has risen by 12%, with overall costs calculated at £3.4 billion.
There were 68,110 children in care on March 31 2013, including 42,228 who had suffered abuse or neglect.
This figure had risen by 12% or 7,210 in the previous four years, according to figures from the Audit Commission.
Councils in England spent £3.4 billion in 2012/13 caring for these vulnerable young people, who represent 0.6% of all under 18-year-olds in England.
Despite this 12% increase, councils' costs rose by 4% nationally, but regional variations ranged from a 15% rise in the North East to a 7% reduction in London.
Spending on foster care totalled £1.5 billion in 2012/13.
An Audit Commission study which looked at council foster care costs and the impact that growing numbers of these children is having on council spending found that 21 councils spent less than £40,000 per child in 2012/13, while 32 councils spent more than £60,000 per child.
The total amount spent on services for these children rose by 69% or £1.4 billion in real terms between 2000/01 and 2012/13.
It was noted that in 2012/13, it accounted for 64% of all the care provided.
Councils' use of foster care increased by a fifth between 2008/09 and 2012/13, and over two-thirds of this extra care was provided by foster care agencies from the private and voluntary sectors.
The cost of agency foster care reduced by 15% during this period but can be more expensive than council-provided care.
Audit Commission chairman Jeremy Newman said: "It is beyond question that councils must place children in settings that meet their individual needs and that provide cost effective, high quality care.
"With pressure to improve outcomes and reduce costs, all councils are faced with the challenge of getting the optimum value from the £137 on average spent per day, which equates to £50,000 a year, looking after each child in their care.
"To meet the complex needs of this group, spending can be more than five times the financial cost of bringing up a child where there is no requirement for council support.
"We encourage all councils to review their spending and in particular urge higher spending councils to understand the reasons for this and to consider whether they can secure more cost-effective placements without compromising on the quality of care."'
Factors such as how easy it is to recruit local foster carers, the availability of suitable local placements when they are needed, the balance between the council's use of its own and agency foster care services and the nature of local foster care services may all affect how much councils spend, according to the Audit Commission's analysis.
Mr Newman suggested that councils should use their collective purchasing power to get maximum value for the £1.5 billion they spend on foster care.
He said: "Rather than competing with each other, potentially driving up prices, councils should consider whether collaborating with neighbouring councils can secure the services they need, at a price they can better afford."
Helen Berresford, of the 4Children charity which is working towards an integrated approach to children's services, pointed out there " is a high financial cost to councils (£3.4 billion) of caring for these vulnerable young people - there is an even greater social and economic cost to not giving these young people the support they need."
The "bleak reality facing some of England's most vulnerable young people" is that children in care are more likely to have mental health problems than their peers, more likely to end up homeless and are also more likely to be excluded from school than other children.
She said: "We need a system of early and intensive support which sets sights high for struggling families and helps reduce the risk of children falling into care.
"Expanding the role of Children's Centres to become community hubs, integrating support services, could deliver the support needed to tackle the underlying causes of crisis for vulnerable families.
"This could be coordinated through Children's Centres, expanded to become community hubs, integrating support services to tackle the many underlying causes of crisis for the most vulnerable families."
A Department for Education spokesman said: "Nothing is more important than keeping children safe. That is why councils have a legal obligation to identify young people who are at risk of harm and intervene.
"The law makes clear that children are best looked-after within their families but every decision to take a young person into care is a matter for the local authority."