A study done by and for the car enthusiast and consumer advice site Edmunds.com calculated that the USA's Federal 'Cash for Clunkers' program cost taxpayers $24,000 for each vehicle sold over if the program had not been affect. Their study calculated that only 125,000 additional cars/SUV were sold, in the about 690,000 total sold under the program. This is a link to their press release on their study. http://www.edmunds.com/help/about/press/159446/article.html
I thought for a long time that instead of a 'trade in' program, they just gave $1000-1500 additional incentives for someone to buy a new higher fuel mileage vehicle, preferably one made in the USA or Canada. You could have sold a lot more cars, not screwed up the scarp metal and car markets, still kept some good cars on the road and not skewed the lower end used car markets.