Also, try to price things more reasonably/realistically. For example, the Ford Mk IV race car is 20,000,000 in GT5. That should be lowered to around 7,000,000 at least.
Maybe the price of the cars should mirror real world prices identically?
Peugeot is putting a 2007 model of their 908 HDi FAP racecar up for auction and it's expected to go for around 2,000,000 which gives an idea of what the prices for modern LMP racecars should be
A McLaren F1 went on auction recently and sold for £2.5 million/$4 million
So maybe split the difference and have the McLaren F1 cost 3,000,000 credits as a used car in GT6. 1,000,000 credits for a McLaren F1 in GT5 grossly undervalued the car even if that was it's original price.
Another example is the Ford GT. It went on sale at $150,000, but the value has gone up and its going for around $200,000 used
edit:
GAMEPLAY: A-Spec/GT Life Features
Real car prices depending on current value (estimated from auctions and used car prices) NOT original selling price. Examples:
8,700,000 credits - 1931 Bugatti Royale Kellner Coupe
4,200,000 credits - Mercedes-Benz 300SL Gullwing
3,200,000 credits - McLaren F1
2,500,000 credits - Ford GT40
2,200,000 credits - Peugeot 908 Race Car
195,000 credits - Ford GT
47,000 credits - Dodge Viper GTS