Assuming the cars are in this game, the "5 most expensive cars" from FM4 are probably the most likely.
I'd guess Shelby Daytona, 250 GTO, and 300SL just based on how rare they are in the real world.
I'm boycotting EA because they butchered the Need for Speed series and refused Turn 10 the Porsche license. Kinect earned the second spot on this list with an achievement in Forza 4 and exclusive features in non-Kinect-Exclusive games like Halo Anniversary.
Forza 4 Flickr Page