well this should have been in the prologe section but, go to the events which have like 35000c or more, and then get in the fastest car possible and do it lots of times. i think i used a-1 (daytona oval) with the ford gt. it is worth geting because if you drive that, then you will become better at the game because your reaction times will improve etc.
or if you cant be bothered (like i wasnt the second time round) just download a save game.
You could also do the 1-Lap F40-Challenge...without cheating (shortcuts), the 3rd place is quite easy and 2nd isnt that hard, either. 2nd place is 48k cr. in about 2,x mins (or how long it takes). Not that bad.
Mines BNR, theres an S class race for this, basically, If your good enough you can get 45k for 2 laps of daytona, thats about 45 races though, if you start from scratch, I started from 600k, still hard work, no matter how you do it.