Been wanting to get a list together of all these, and thanks to someone at GTP, most of the work was done for me.
In no particular order...
McLaren F1 GTR '95...............................$1,750,000
Peugeot 908 HDI FAP '10........................$1,900,000
Renault Sport Megane Trophy '11.............$100,000
Autobacs Garaiya GT300 '08....................$500,000
Honda NSX GT500 '08............................$950,000
Honda HSV-010 GT '12...........................$980,000
Lexus IS350 GT300 '08...........................$500,000
Lexus SC430 GT500 '08..........................$950,000
Monster Sport SX4 '11...........................$1,600,000
Nissan GT-R GT500 '08...........................$950,000
Nissan Z GT500 '06................................$950,000
Nissan GT-R NISMO GT3 '13.....................$350,000
RE Amemiya RX7 GT300 '06.....................$500,000
Subaru Impreza GT300 '08......................$500,000
Subaru BRZ GT300 '12...........................$500,000
Toyota 86 Race Car '12.........................$50,000
Toyota Supra GT500 '05........................$900,000
Volkswagen Scirocco GT24 '08................$420,000
Hidden bonus car...
BMW M3 GT2 '11.................................$350,000
This car won't show up in the dealership, unless you run the full race version(loaner) in Arcade Mode. I don't know if it matters where, but GTP users were running Nurb, which I did as well. Here's the kicker, you don't even have to finish the race. Just start it, back out, and the Base model will appear in the dealership.
Grand total cost for the above..............$14,700,000
I'm going to work on getting a price list of RMs Touring Cars together too.