Many used cars have gotten more expensive, but some secondhand sports have remained surprisingly cheap. However, when it comes to the BMW E60 M5, E63/E64 M6, E46 and E92 M3, Audi B6/B7 S4, Mercedes CL 63 AMG, Jaguar XK8, and Mazda RX-8, what's cheap to buy isn't cheap to maintain.