If you're new at the game, though, there are still easy ways to get money that don't involve repeating races, such as:
Get the 16,900cr Miata.
Win Sunday Cup.
Get the Stage 1 Turbo (132 HP -> 250 HP)
Win Clubman, FR Challenge and Spider & Roadster easily.
At that point, you can take the Miata LS prize car from Spider & Roadster, put T2's on it and win Roadster Endurance by a few laps, which gives you 100,000cr and possibly an F1 car.
You could also get a few more upgrades and win the Beginner GT World at that point, which can give you a free Skyline with can be upgraded to 850 HP pretty cheap.
If you really just want a bunch of money, there are plenty races you can do for a ton of cash once you get an OK car. A popular choice is the Amateur European Championship which gives you 350,000cr total (300,000 from the Vertigo, 7,500 per race, 20,000 series bonus). That's pretty good, and you only have to do four 3-lap races then do the save/load thing and quit. 350,000cr for 12 laps! There are plenty other things you should do before you re-race it for money, though.
Amateur All-Japan gives you 612,500cr total (312,500 for each prize car, 15,000 per race, 150,000 series bonus). It's ten 5-lap races, but it's more money and you don't have to do the save/load trick because all the cars are worth the same. If you're going to sell the prize car:
Amateur All-Star gives up to 750,000cr (ten 5-lap races).
Amateur GT World gives up to 750,000cr (ten 5-lap races).
Pro Dream Car gives up to 1,030,000cr (seven 15-lap races).
Pro GT World gives up to 1,150,000cr (ten 10-lap races).
Pro All-Star gives up to 1,400,000cr (ten 15-lap races).
PD Cup gives up to 1,825,000cr (although you probably wouldn't sell the F094/S).
but thanks for telling me well to answer the other guys question, I can't beat the euro championship with a stock miata or sprinter. Notice the big caps that say this is for beginners
