To conduct the analysis, the Edmunds.com team of PhDs and statisticians examined the sales trend for luxury vehicles and others not included in Cash for Clunkers, and applied the historic relationship of those vehicles to total SAAR to make informed estimates. These estimates were independently verified through careful examination of sales patterns reflected by transaction data. Once the numbers were determined, Edmunds.com's analysts divided three billion dollars by 125,000 vehicles to arrive at the average $24,000 per vehicle.
Cash for Clunkers Results Finally In: Taxpayers Paid $24,000 per Vehicle Sold, Reports Edmunds.com
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ps - to answer Goldenboy's question, I couldn't find it.