From CNN Money:
The Cash for Clunkers program gave car buyers rebates of up to $4,500 if they traded in less fuel-efficient vehicles for new vehicles that met certain fuel economy requirements. A total of $3 billion was allotted for those rebates.
The average rebate was $4,000. But the overwhelming majority of sales would have taken place anyway at some time in the last half of 2009, according to Edmunds.com. That means the government ended up spending about $24,000 each for those 125,000 additional vehicle sales.