A couple of economists have calculated the Cash for Clunkers is costing us anywhere from $200-500 per ton of carbon dioxide saved. This as opposed to the estimated $28/ton that the climate bill will cost. I think both their estimates understates the cost and overstates the pollution savings, since you also have to take into account that cars which, while creaky, still have useful lives are prematurely junked. Last I checked, melting down an old car and building a new one cost money and produce pollutants.
Added bonus: our auto industry, which should be slimming down, gets a false burst of demand which means resources that should be moving to industries that can use them more efficiently stay trapped within the Detroit boondongle.
NY Times article here.