If you Patient
Do a 23 Lap Race on Grand Canyon in S Rank
300,000k..
My choice off Car is the Ford RS200
(Trying to save up for the Ferrari F1 Car atm)
Edit: I assume you meant Cr300,000 not 300,000 * Cr1000 or else saving for the Ferrari F1 would not be an issue.
Assuming you mean a race race and not a drift race, if you did several 4 or 5 lap races you'd make Cr150,000 for each, so about Cr600,000 for about the same time, and no more game days as long as you don't exit from the post-race screen.
Edit: Sorry, actually 6 laps is the magic number for Grand Canyon, so you'd need 24 laps for Cr600,000.
It seems that Cr50,000 for D (corresponds to Cr150,000 for S) is the optimal payout.
Nürburgring - 1 lap (fewest laps)
Test Course - 2 laps (shortest time)
Super Speedway - 9 laps (close to shortest time)
High Speed Ring - 5 laps not to far from shortest, lets you learn a little cornering (reverse might be easier to win?)
Those Cr50,000/Cr150,000 races in general seem to take 8 to 10 minutes. Although a lot less time on high-speed tracks.
In general, a good way to make progress and money is to pick a car which will do well at a track (perhap use single model race to help), select the Cr50,000 number of laps, and unlock that track to S, and perhaps win a couple of S races, all in one game day. I seem to subconsciously learn to drive better. (how about that) D is almost "free practice" with money. But with some cars/tracks I just don't seem to do so well.
Although, does anyone else think that the initial D (so as to speak) is even dumber than D if you select it later? As if the AI level is either its current max, or capped at the level you pick. And each letter can be a range of values, so as you race that level gets smarter until you unlock the next level.
Of course, if it turns out your car and track combination are troublesome, you won't make the progress or money you had hoped for. (If you have Autosave off, you can reload and redo the day, of course).