It's always been a bit of slog to buy cars in Gran Turismo. However, with Gran Turismo 6, Sony will let you buy in-game credits with real-world money.
In a post on the PlayStation EU Blog, European brand manager Penrose Tackie announced that you'll be able to buy in-game credits in various denominations: 500,000, 1 million, 2.5 million, or 7 million. You can then use those credits to buy cars or parts a la carte. The post didn't detail the price of each denomination pack. It did, however, note that for 1 million credits, you could pick up nine high-end cars, including the BMW Z4 GT3 11, Ford GT, and Tesla Model S.