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It depends on what you mean. If the question is referring to new production cars, it would be the Bugatti Veyron 16.4, with a price tag of about $1.2 million (US), according to Forbes. See more about the bugatti at the Bugatti wiki
The most expensive car ever auctioned off was a 1931 Bugatti Royale Kellner Coupe, for $8.7 million (US) by Christies of London.
Of course, concept cars can run into the range of several million dollars. Though not for sale, the amount of money spent on research and development on concept cars can easily tip the scales into the $5-10 million range.
If you were to include military vehicles as well, the amount spent on R&D can go much higher (remember NASA's $100,000 crescent wrench?). Though few would consider the US Abrams M1A1 tank as a "car", they cost about $4.3 million a pop.