The Times’s “Green, Inc.” blog has caught on that cash-for-clunkers is not exxactly the cheapest emissions-reduction program, as Ed noted here:

Whatever its success in getting new cars off the lots and quickly injecting several billion dollars into the economy, some academics say that the “cash for clunkers” program is an expensive way to help the environment.
The popular program may be costing the government — and taxpayers — several hundred dollars for each ton of carbon dioxide emissions that it saves, according to conclusions reached separately by Michael Wara at Stanford University and Christopher R. Knittel of the the University of California, Davis. Both performed back-of-the envelope calculations to figure out how well the program was meeting its environmental goals.
“The program is really not cost effective as a climate policy,” said Mr. Wara, who is an assistant professor at Stanford Law School and a faculty fellow at the university’s program on energy and sustainable development. “It might be a great economic stimulus — we’re selling a lot of cars — but this is not the way to deal with mobile sources of climate change.”