I can't remember exactly what license you need but participate in Round 3 of GT All-Stars at Red Rock Valley.
If you win you get 25000Cr - I think - and the TVR Speed 12 which can be sold for 500,000Cr. Not bad for just 5 laps work
BTW - If you do try the Speed 12 - which I advise - be very careful with the accelerator. I've got a pretty decent Speed 12 setup for Red Rock Valley which can produce times between 58 - 59 seconds. If interested give me a shout...