Price system in GT7 is good it is just earning credits broken.
I respectfully dissagree, there's too large a gap between the lower priced cars and the higher priced cars in my opinion. The economy is broken with it taking so long to save to buy the expensive cars, but if you made it possible to earn 10m Cr in an hour you would break the game economy in a different way.
You need a games economy to be balanced so that in the end game, no individual car takes too long to save up for, but you aren't earning so many credits that you can buy almost everything else in moments either. GT2 for example broke it's economy with a single race, the GT All Stars at Red Rock Valley.
Although the prize Cr is only 50k, you won a car you could sell for 500k, for 5 laps of a fast track. 5 laps, roughly 5 minutes of racing, represented earning 27.5% of the most expensive car in the games cost. That was too fast and that event just became the go-to grind to save up. All GT games have had thier go to grind events, the DTM series in GT4 for example gave you a car worth over 700k if you won, but none quite so big as GT2's.
If it was me, and granted, this is just me, everyone will have differing sweet spots, I would set the game so it takes a good while to reach the end game stage. There would be a lot of events and races to clear and these would gradually increase in difficulty and earnings per minute of racing.
So at the start, you'd see a 2m credit car and it'd seem almost unobtainable at the rate you are earning credits early on. By the end game though, you're earning 2m credits in a couple of hours, but since it's taken you a long time to get there it still feels rewarding. Because the cars are all a 2m Cr range, earning 1m Cr in an hour isn't so obscene that it removes having to save for anything other than the most expensive cars, whilst at the same time, prevents the player from having to grind seemingly endlessly to purchase one car.
The races should be based on the difficulty, the stage of the game they are unlocked and a cr per minute time forumla. Of course, you can't do an exact, but you can work out an approximate time each race should take and then calcualte the credits awarded from that.
There's no real shortage of cars in GT7, and the body kits and livery editor makes it really appealing to have multiple of the same car. So it's not like making the economy more balanced would stop people having anything to save for. I'm sure some people would love to have a custom races with a grids full of some of the more expensive cars but will never get the chance due to the time it takes to achieve that and/or the dull grind involved that disengages players.
You simply make the single player career, lengthy but also rewarding, with defined goals, and instead of grinding the same 3 races over and over and over again, by the time you've reached the end point, you're in a position where saving for those end game cars doesn't require putting your life on hold.